diff --git a/README.md b/README.md
index 949ee65..f0c4c32 100644
--- a/README.md
+++ b/README.md
@@ -696,9 +696,12 @@ user's return. This will be at your `returnUrl` endpoint:
 
 ```php
 // The result will be read and decrypted from the return URL (or failure URL)
-// query parameters:
+// query parameters.
+// You MUST provide the original expected transactionId, which is validated
+// against the transactionId provided in the server request.
+// This prevents different payments getting mixed up.
 
-$result = $gateway->completeAuthorize()->send();
+$result = $gateway->completeAuthorize(['transactionId' => $originalTransactionId])->send();
 
 $result->isSuccessful();
 $result->getTransactionReference();
diff --git a/src/Message/Form/CompleteAuthorizeRequest.php b/src/Message/Form/CompleteAuthorizeRequest.php
index 6251e48..420c5a8 100644
--- a/src/Message/Form/CompleteAuthorizeRequest.php
+++ b/src/Message/Form/CompleteAuthorizeRequest.php
@@ -9,6 +9,7 @@
 use Omnipay\SagePay\Message\AbstractRequest;
 use Omnipay\SagePay\Message\Response as GenericResponse;
 use Omnipay\Common\Exception\InvalidResponseException;
+use Omnipay\Common\Exception\InvalidRequestException;
 
 class CompleteAuthorizeRequest extends AbstractRequest
 {
@@ -72,14 +73,37 @@ public function getData()
 
     /**
      * Nothing to send to gateway - we have the result data in the server request.
+     *
+     * @throws InvalidResponseException
+     * @throws InvalidResponseException
      */
     public function sendData($data)
     {
+        $this->response = new GenericResponse($this, $data);
+
+        // Issue #131: confirm the response is for the transaction ID we are
+        // expecting, and not replayed from another transaction.
+
+        $originalTransactionId = $this->getTransactionId();
+        $returnedTransactionId = $this->response->getTransactionId();
+
+        if (empty($originalTransactionId)) {
+            throw new InvalidRequestException('Missing expected transactionId parameter');
+        }
+
+        if ($originalTransactionId !== $returnedTransactionId) {
+            throw new InvalidResponseException(sprintf(
+                'Unexpected transactionId; expected "%s" received "%s"',
+                $originalTransactionId,
+                $returnedTransactionId
+            ));
+        }
+
         // The Response in the current namespace conflicts with
         // the Response in the namespace one level down, but only
         // for PHP 5.6. This alias works around it.
 
-        return $this->response = new GenericResponse($this, $data);
+        return $this->response;
     }
 
     /**
diff --git a/tests/FormGatewayTest.php b/tests/FormGatewayTest.php
index a9f6cc2..8f141b5 100644
--- a/tests/FormGatewayTest.php
+++ b/tests/FormGatewayTest.php
@@ -34,6 +34,7 @@ public function setUp()
 
         $this->completePurchaseOptions = [
             'encryptionKey' => '2f52208a25a1facf',
+            'transactionId' => 'phpne-demo-53922585',
         ];
     }
 
@@ -235,7 +236,12 @@ public function testCompletePurchaseSuccess()
 
         $this->getHttpRequest()->initialize(['crypt' => '@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']);
 
-        $response = $this->gateway->completePurchase($this->completePurchaseOptions)->send();
+        $options = $this->completePurchaseOptions;
+
+        // Switch to the transaction ID actually encrypted in the server request.
+        $options['transactionId'] = 'phpne-demo-56260425';
+
+        $response = $this->gateway->completePurchase($options)->send();
 
         $this->assertTrue($response->isSuccessful());
         $this->assertFalse($response->isRedirect());
@@ -266,4 +272,44 @@ public function testCompletePurchaseSuccess()
             $response->getData()
         );
     }
+
+    /**
+     * The wrong transaction ID is supplied with the server request.
+     *
+     * @expectedException Omnipay\Common\Exception\InvalidResponseException
+     */
+    public function testCompletePurchaseReplayAttack()
+    {
+        //$this->expectException(Complicated::class);
+
+        // Set the "crypt" query parameter.
+
+        $this->getHttpRequest()->initialize(['crypt' => '@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']);
+
+        // These options contain a different transactionId from the once expected.
+
+        $options = $this->completePurchaseOptions;
+
+        $response = $this->gateway->completePurchase($options)->send();
+    }
+
+    /**
+     * The missing expected transaction ID supplied by the app.
+     *
+     * @expectedException Omnipay\Common\Exception\InvalidRequestException
+     */
+    public function testCompletePurchaseMissingExpectedParam()
+    {
+        //$this->expectException(Complicated::class);
+
+        // Set the "crypt" query parameter.
+
+        $this->getHttpRequest()->initialize(['crypt' => '@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']);
+
+        $options = $this->completePurchaseOptions;
+
+        unset($options['transactionId']);
+
+        $response = $this->gateway->completePurchase($options)->send();
+    }
 }