diff --git a/packages/light-react/package.json b/packages/light-react/package.json
index 11d54f773d577859ee369cdfad985eadd86e4a58..9e2ee97715a5929a93e0e77bf0a14a1f4c573dd1 100644
--- a/packages/light-react/package.json
+++ b/packages/light-react/package.json
@@ -38,7 +38,7 @@
     "@parity/api": "^2.1.22",
     "@parity/light.js": "https://github.com/parity-js/light.js#9646ce15d9dd9c4cf11776ddd613d5bd86016f94",
     "@parity/shared": "^3.0.2",
-    "bignumber.js": "^7.2.1",
+    "bignumber.js": "^4.1.0",
     "is-electron": "^2.1.0",
     "light-hoc": "^0.1.0",
     "light-ui": "^0.1.0",
@@ -50,6 +50,7 @@
     "react-dom": "^16.3.2",
     "react-router-dom": "^4.2.2",
     "react-scripts": "1.1.4",
+    "react-tooltip": "^3.6.1",
     "rxjs": "^6.2.0"
   },
   "devDependencies": {
diff --git a/packages/light-react/src/Onboarding/Onboarding.js b/packages/light-react/src/Onboarding/Onboarding.js
index aa2edcc2c13f9361ecd6990b9c7ad73fe985d8c6..da9f981bc9a66e92ff48255772ac641bfe188729 100644
--- a/packages/light-react/src/Onboarding/Onboarding.js
+++ b/packages/light-react/src/Onboarding/Onboarding.js
@@ -32,7 +32,7 @@ class Onboarding extends Component {
                   readOnly
                 />
               }
-              label="Please read carefully Fether Wallet's Terms of Use"
+              label='Please read carefully'
             />
           </div>
         </div>
diff --git a/packages/light-react/src/Send/Signer/Signer.js b/packages/light-react/src/Send/Signer/Signer.js
index 91c9cfb78399311e85fa2a03f20cc3291d08926e..5c0ecdb91955021473c2f93b12f36a7800f3ceb7 100644
--- a/packages/light-react/src/Send/Signer/Signer.js
+++ b/packages/light-react/src/Send/Signer/Signer.js
@@ -4,9 +4,11 @@
 // SPDX-License-Identifier: MIT
 
 import React, { Component } from 'react';
+import { findDOMNode } from 'react-dom';
 import { FormField, Header } from 'light-ui';
 import { inject, observer } from 'mobx-react';
 import { Link } from 'react-router-dom';
+import ReactTooltip from 'react-tooltip';
 
 import TokenBalance from '../../Tokens/TokensList/TokenBalance';
 
@@ -14,26 +16,31 @@ import TokenBalance from '../../Tokens/TokensList/TokenBalance';
 @observer
 class Signer extends Component {
   state = {
+    error: null,
     isSending: false,
     password: ''
   };
 
-  handleAccept = e => {
+  handleAccept = event => {
     const { history, sendStore } = this.props;
     const { password } = this.state;
 
-    e.preventDefault();
+    event.preventDefault();
 
     this.setState({ isSending: true }, () => {
       sendStore
         .acceptRequest(password)
         .then(() => history.push('/send/sent'))
-        .catch(() => this.setState({ isSending: false }));
+        .catch(error => {
+          this.setState({ error, isSending: false }, () =>
+            ReactTooltip.show(findDOMNode(this.tooltip))
+          );
+        });
     });
   };
 
   handleChangePassword = ({ target: { value } }) => {
-    this.setState({ password: value });
+    this.setState({ error: null, password: value });
   };
 
   handleReject = () => {
@@ -47,11 +54,15 @@ class Signer extends Component {
     });
   };
 
+  handleTooltipRef = ref => {
+    this.tooltip = ref;
+  };
+
   render () {
     const {
       sendStore: { token, tx, txStatus }
     } = this.props;
-    const { isSending, password } = this.state;
+    const { error, isSending, password } = this.state;
 
     return (
       <div>
@@ -85,13 +96,18 @@ class Signer extends Component {
                     <p>Enter your password to confirm this transaction.</p>
                   </div>
 
-                  <FormField
-                    label='Password'
-                    onChange={this.handleChangePassword}
-                    required
-                    type='password'
-                    value={password}
-                  />
+                  <div
+                    data-tip={error ? error.text : ''}
+                    ref={this.handleTooltipRef}
+                  >
+                    <FormField
+                      label='Password'
+                      onChange={this.handleChangePassword}
+                      required
+                      type='password'
+                      value={password}
+                    />
+                  </div>
 
                   <nav className='form-nav -binary'>
                     <button
@@ -101,9 +117,12 @@ class Signer extends Component {
                     >
                       Cancel
                     </button>
+
                     <button
                       className='button -submit'
-                      disabled={!txStatus || !txStatus.requested || isSending}
+                      disabled={
+                        !txStatus || !txStatus.requested || isSending || error
+                      }
                     >
                       Confirm transaction
                     </button>
@@ -115,6 +134,12 @@ class Signer extends Component {
             />
           </div>
         </div>
+        <ReactTooltip
+          effect='solid'
+          event='mouseover'
+          eventOff='keydown mouseout'
+          place='top'
+        />
       </div>
     );
   }
diff --git a/packages/light-react/src/Send/TxForm/TxForm.js b/packages/light-react/src/Send/TxForm/TxForm.js
index 3c4d90b2d946ae19f19c3399083f9f468e3777d1..ab8baaf419d5dd63405e5893db6e3073ba337568 100644
--- a/packages/light-react/src/Send/TxForm/TxForm.js
+++ b/packages/light-react/src/Send/TxForm/TxForm.js
@@ -4,10 +4,13 @@
 // SPDX-License-Identifier: MIT
 
 import React, { Component } from 'react';
+import debounce from 'lodash/debounce';
 import { FormField, Header } from 'light-ui';
 import { fromWei, toWei } from '@parity/api/lib/util/wei';
 import { inject, observer } from 'mobx-react';
+import { isAddress } from '@parity/api/lib/util/address';
 import { Link } from 'react-router-dom';
+import ReactTooltip from 'react-tooltip';
 
 import TokenBalance from '../../Tokens/TokensList/TokenBalance';
 import withBalance from '../../utils/withBalance';
@@ -19,42 +22,54 @@ const MIN_GAS_PRICE = 3; // Safelow gas price from GasStation, in Gwei
 @withBalance(({ sendStore: { token } }) => token)
 @observer
 class Send extends Component {
-  componentDidMount () {
-    this.props.sendStore.estimateGas();
+  state = {
+    amount: '', // In Ether or in token
+    gasPrice: 4, // in Gwei
+    to: '',
+    ...this.props.sendStore.tx
+  };
+
+  componentDidUpdate () {
+    if (!this.hasError()) {
+      const { amount, gasPrice, to } = this.state;
+      this.props.sendStore.setTx({ amount, gasPrice, to });
+      this.estimateGas();
+    }
   }
 
-  getMaxAmount = () => {
+  estimateGas = debounce(() => {
+    this.props.sendStore.estimateGas();
+  }, 1000);
+
+  static getDerivedStateFromProps (nextProps, prevState) {
     const {
       balance,
-      sendStore: { estimated, tx }
-    } = this.props;
-
-    // TODO this in sendStore as @computed?
-    return balance && estimated
-      ? +fromWei(
-        toWei(balance).minus(
-          estimated.multipliedBy(toWei(tx.gasPrice, 'shannon'))
-        )
-      )
-      : 0.01;
-  };
+      sendStore: { estimated }
+    } = nextProps;
+
+    return {
+      maxAmount:
+        balance && estimated
+          ? +fromWei(
+            toWei(balance).minus(
+              estimated.mul(toWei(prevState.gasPrice, 'shannon'))
+            )
+          )
+          : 0.01
+    };
+  }
 
   handleChangeAmount = ({ target: { value } }) =>
-    this.props.sendStore.setTxAmount(value);
+    this.setState({ amount: value });
 
   handleChangeGasPrice = ({ target: { value } }) =>
-    this.props.sendStore.setTxGasPrice(value);
+    this.setState({ gasPrice: value });
 
   handleChangeTo = ({ target: { value } }) => {
-    const { sendStore } = this.props;
-    sendStore.setTxTo(value);
-    // Estimate the gas to this address, if we're sending ETH.
-    if (sendStore.tokenAddress === 'ETH') {
-      sendStore.estimateGas();
-    }
+    this.setState({ to: value });
   };
 
-  handleMax = () => this.props.sendStore.setTxAmount(this.getMaxAmount());
+  handleMax = () => this.setState({ amount: this.state.maxAmount });
 
   handleSubmit = e => {
     e.preventDefault();
@@ -67,10 +82,39 @@ class Send extends Component {
     history.push('/send/signer');
   };
 
+  /**
+   * Get form errors.
+   *
+   * TODO Use a React form library to do this?
+   */
+  hasError = () => {
+    const { amount, maxAmount, to } = this.state;
+    if (!amount || isNaN(amount)) {
+      return 'Please enter a valid amount';
+    }
+
+    if (amount < 0) {
+      return 'Please enter a positive amount ';
+    }
+
+    if (amount > maxAmount) {
+      return "You don't have enough balance";
+    }
+
+    if (!isAddress(to)) {
+      return 'Please enter a valid Ethereum address';
+    }
+
+    return null;
+  };
+
   render () {
     const {
-      sendStore: { token, tx }
+      sendStore: { token }
     } = this.props;
+    const { amount, gasPrice, maxAmount, to } = this.state;
+
+    const error = this.hasError();
 
     return (
       <div>
@@ -99,14 +143,14 @@ class Send extends Component {
                         <div>
                           <input
                             className='form_field_amount'
-                            max={this.getMaxAmount()}
+                            formNoValidate
+                            max={maxAmount}
                             min={0}
                             onChange={this.handleChangeAmount}
-                            placeholder='1.00'
                             required
-                            step={10 ** -token.decimals}
+                            step={+fromWei(1)}
                             type='number'
-                            value={tx.amount}
+                            value={amount}
                           />
                           <nav className='form-field_nav'>
                             <button
@@ -124,15 +168,13 @@ class Send extends Component {
 
                     <FormField
                       input={
-                        <input
+                        <textarea
                           className='-sm'
                           onChange={this.handleChangeTo}
-                          pattern='^0x[a-fA-F0-9]{40}$'
                           placeholder='0x...'
                           required
-                          title='Invalid Ethereum address'
                           type='text'
-                          value={tx.to}
+                          value={to}
                         />
                       }
                       label='To'
@@ -149,12 +191,12 @@ class Send extends Component {
                             required
                             step={0.5}
                             type='range'
-                            value={tx.gasPrice}
+                            value={gasPrice}
                           />
                           <nav className='range-nav'>
                             <span className='range-nav_label'>Cheap</span>
                             <span className='range-nav_value'>
-                              {tx.gasPrice} Gwei
+                              {gasPrice} Gwei
                             </span>
                             <span className='range-nav_label'>Fast</span>
                           </nav>
@@ -164,7 +206,11 @@ class Send extends Component {
                     />
                   </fieldset>
                   <nav className='form-nav'>
-                    <button className='button'>Send</button>
+                    <span data-tip={error || ''}>
+                      <button disabled={error} className='button'>
+                        Send
+                      </button>
+                    </span>
                   </nav>
                 </form>
               ]}
@@ -173,6 +219,12 @@ class Send extends Component {
             />
           </div>
         </div>
+        <ReactTooltip
+          effect='solid'
+          event='mouseover'
+          eventOff='mouseout'
+          place='top'
+        />
       </div>
     );
   }
diff --git a/packages/light-react/src/assets/sass/shared/_button.scss b/packages/light-react/src/assets/sass/shared/_button.scss
index f03bf5cf79a67f483ba1c16f899c4d7078829181..c593219782bed3efa936f396266bdfbc54752ded 100644
--- a/packages/light-react/src/assets/sass/shared/_button.scss
+++ b/packages/light-react/src/assets/sass/shared/_button.scss
@@ -24,6 +24,7 @@
     border-color: $faint;
     background-color: $faint;
     color: $grey;
+    pointer-events: none;
   }
 
   &.-tiny {
diff --git a/packages/light-react/src/stores/sendStore.js b/packages/light-react/src/stores/sendStore.js
index fea2d9938520af8d9b85ff148b2dfa73cd80ebc5..da4d45272c925b3b82bc41da0fd5818256659a0d 100644
--- a/packages/light-react/src/stores/sendStore.js
+++ b/packages/light-react/src/stores/sendStore.js
@@ -7,7 +7,7 @@ import abi from '@parity/shared/lib/contracts/abi/eip20';
 import { action, computed, observable } from 'mobx';
 import { BigNumber } from 'bignumber.js';
 import { blockNumber$, makeContract$, post$ } from '@parity/light.js';
-import { isAddress } from '@parity/api/lib/util/address';
+import memoize from 'lodash/memoize';
 import noop from 'lodash/noop';
 import { toWei } from '@parity/api/lib/util/wei';
 
@@ -15,17 +15,13 @@ import parityStore from './parityStore';
 import tokensStore from './tokensStore';
 
 const DEFAULT_GAS = new BigNumber(21000); // Default gas amount to show
+const GAS_MULT_FACTOR = 1.2; // Since estimateGas is not always accurate, we add a 120% factor for buffer.
 
 class SendStore {
   @observable blockNumber; // Current block number, used to calculate tx confirmations.
   @observable estimated = DEFAULT_GAS; // Estimated gas amount for this transaction.
   @observable tokenAddress; // 'ETH', or the token contract address
-  @observable
-  tx = {
-    amount: '', // In Ether or in token
-    gasPrice: 4, // in Gwei
-    to: ''
-  }; // The actual tx we are sending. No need to be observable.
+  tx = {}; // The actual tx we are sending. No need to be observable.
   @observable txStatus; // Status of the tx, see wiki for details.
 
   constructor () {
@@ -84,47 +80,39 @@ class SendStore {
    * Estimate the amount of gas for our transaction.
    */
   estimateGas = () => {
-    if (!this.isTxValid) {
-      return Promise.resolve(DEFAULT_GAS);
-    }
-
     if (this.tokenAddress === 'ETH') {
-      return this.api.eth
-        .estimateGas(this.txForEth)
-        .then(this.setEstimated)
-        .catch(noop);
+      return this.estimateGasForEth(this.txForEth);
     } else {
-      return this.contract.contractObject.instance.transfer
-        .estimateGas(this.txForErc20.options, this.txForErc20.args)
-        .then(this.setEstimated)
-        .catch(noop);
+      return this.estimateGasForErc20(this.txForErc20);
     }
   };
 
-  @computed
-  get isTxValid () {
-    if (
-      !this.tx || // There should be a tx
-      !isAddress(this.tx.to) || // The address should be okay
-      !this.tx.amount ||
-      isNaN(this.tx.amount) ||
-      isNaN(this.tx.gasPrice)
-    ) {
-      return false;
-    }
+  /**
+   * Estimate gas to transfer in ERC20 contract. Expensive function, so we
+   * memoize it.
+   */
+  estimateGasForErc20 = memoize(txForErc20 => {
+    return this.contract.contractObject.instance.transfer
+      .estimateGas(txForErc20.options, txForErc20.args)
+      .then(this.setEstimated)
+      .catch(noop);
+  }, JSON.stringify);
 
-    return true;
-  }
+  /**
+   * Estimate gas to transfer to an ETH address. Expensive function, so we
+   * memoize it.
+   */
+  estimateGasForEth = memoize(txForEth => {
+    return this.api.eth
+      .estimateGas(txForEth)
+      .then(this.setEstimated)
+      .catch(noop);
+  }, JSON.stringify);
 
   /**
    * Create a transaction.
    */
   send = () => {
-    if (!this.isTxValid) {
-      console.error('Transaction is invalid.', this.tx);
-      return;
-    }
-
     const send$ =
       this.tokenAddress === 'ETH'
         ? post$(this.txForEth)
@@ -156,37 +144,16 @@ class SendStore {
     return tokensStore.tokens[this.tokenAddress];
   }
 
-  /**
-   * This.tx is a user-friendly tx object. We convert it now as it can be
-   * passed to post$(tx).
-   */
-  @computed
-  get txForEth () {
-    if (!this.isTxValid) {
-      return {};
-    }
-
-    return {
-      gasPrice: toWei(this.tx.gasPrice, 'shannon'), // shannon == gwei
-      to: this.tx.to,
-      value: toWei(this.tx.amount.toString())
-    };
-  }
-
   /**
    * This.tx is a user-friendly tx object. We convert it now as it can be
    * passed to makeContract$(...).
    */
   @computed
   get txForErc20 () {
-    if (!this.isTxValid) {
-      return {};
-    }
-
     return {
       args: [
         this.tx.to,
-        new BigNumber(this.tx.amount).multipliedBy(
+        new BigNumber(this.tx.amount).mul(
           new BigNumber(10).pow(this.token.decimals)
         )
       ],
@@ -196,42 +163,37 @@ class SendStore {
     };
   }
 
-  @action
-  setTokenAddress = tokenAddress => {
-    this.tokenAddress = tokenAddress;
-  };
-
-  @action
-  setTx = tx => {
-    this.tx = tx;
-  };
-
-  @action
-  setTxAmount = amount => {
-    this.tx.amount = amount;
-  };
+  /**
+   * This.tx is a user-friendly tx object. We convert it now as it can be
+   * passed to post$(tx).
+   */
+  @computed
+  get txForEth () {
+    return {
+      gasPrice: toWei(this.tx.gasPrice, 'shannon'), // shannon == gwei
+      to: this.tx.to,
+      value: toWei(this.tx.amount.toString())
+    };
+  }
 
   @action
-  setTxGasPrice = gasPrice => {
-    this.tx.gasPrice = gasPrice;
+  setBlockNumber = blockNumber => {
+    this.blockNumber = blockNumber;
   };
 
   @action
-  setTxTo = to => {
-    this.tx.to = to;
+  setEstimated = estimated => {
+    this.estimated = estimated.mul(GAS_MULT_FACTOR);
   };
 
   @action
-  setBlockNumber = blockNumber => {
-    this.blockNumber = blockNumber;
+  setTokenAddress = tokenAddress => {
+    this.tokenAddress = tokenAddress;
   };
 
   @action
-  setEstimated = estimated => {
-    // Since estimateGas is not always accurate, we add a 120% factor for buffer.
-    const GAS_MULT_FACTOR = 1.2;
-
-    this.estimated = estimated.multipliedBy(GAS_MULT_FACTOR);
+  setTx = tx => {
+    this.tx = tx;
   };
 
   @action
diff --git a/packages/light-ui/src/FormField/FormField.js b/packages/light-ui/src/FormField/FormField.js
index e1e337192b1625e9591c52499c8e76dca6f3ce1c..b2f75e8295414c86dbdabb81ea49d7afcc81a713 100644
--- a/packages/light-ui/src/FormField/FormField.js
+++ b/packages/light-ui/src/FormField/FormField.js
@@ -6,10 +6,15 @@
 import React from 'react';
 import PropTypes from 'prop-types';
 
-const FormField = ({ className, input, label, ...otherProps }) => (
+const FormField = ({
+  className,
+  input: inputComponent,
+  label,
+  ...otherProps
+}) => (
   <div className={['form_field', className].join(' ')}>
     <label>{label}</label>
-    {input || <input {...otherProps} />}
+    {inputComponent || <input {...otherProps} />}
   </div>
 );
 
diff --git a/yarn.lock b/yarn.lock
index b6dda5a6bfb09bee9760a8bfaa5dc9d49475832f..4cb38944c44e2eb22f416089f1ef3a2593690190 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2468,14 +2468,10 @@ big.js@^3.1.3:
   version "3.2.0"
   resolved "https://registry.yarnpkg.com/big.js/-/big.js-3.2.0.tgz#a5fc298b81b9e0dca2e458824784b65c52ba588e"
 
-bignumber.js@4.1.0:
+bignumber.js@4.1.0, bignumber.js@^4.1.0:
   version "4.1.0"
   resolved "https://registry.yarnpkg.com/bignumber.js/-/bignumber.js-4.1.0.tgz#db6f14067c140bd46624815a7916c92d9b6c24b1"
 
-bignumber.js@^7.2.1:
-  version "7.2.1"
-  resolved "https://registry.yarnpkg.com/bignumber.js/-/bignumber.js-7.2.1.tgz#80c048759d826800807c4bfd521e50edbba57a5f"
-
 binary-extensions@^1.0.0:
   version "1.11.0"
   resolved "https://registry.yarnpkg.com/binary-extensions/-/binary-extensions-1.11.0.tgz#46aa1751fb6a2f93ee5e689bb1087d4b14c6c205"