diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7746fc8..a61c0df 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,11 +5,14 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
+### Changed
+- Revert support for token auth, added in v6.1.0.
+- Label username and password for clientId and clientSecret. No param changes.
 
 ## [6.1.0] - 2025-06-25
 ### Added
-- Support for API token authentication. Username and password can still be used
-  as a fallback.
+- ~~Support for API token authentication. Username and password can still be used
+  as a fallback.~~ Reverted in v6.1.1.
 ### Changed
 - Deprecate `getConfig()` method. Packages can maintain their config internally.
 
@@ -21,10 +24,15 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - **BC break**: Removed support for PHP versions <= v8.0 as they are no longer
   [actively supported](https://php.net/supported-versions.php) by the PHP project.
 
+## [5.2.1] - 2025-07-20
+### Changed
+- Revert support for token auth, added in v5.2.0.
+- Label username and password for clientId and clientSecret. No param changes.
+
 ## [5.2.0] - 2025-06-25
 ### Added
-- Support for API token authentication. Username and password can still be used
-  as a fallback.
+- ~~Support for API token authentication. Username and password can still be used
+  as a fallback.~~ Reverted in v5.2.1.
 ### Changed
 - Deprecate `getConfig()` method. Packages can maintain their config internally.
 
diff --git a/README.md b/README.md
index 485f8af..2f9cb7c 100644
--- a/README.md
+++ b/README.md
@@ -36,7 +36,8 @@ $ composer require maxemail/api-php
 ```php
 // Instantiate Client:
 $config = [
-    'token' => 'apitoken',
+    'username' => 'client ID',
+    'password' => 'client secret'
 ];
 $api = new \Maxemail\Api\Client($config);
 
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index c313011..2853b4a 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -12,7 +12,8 @@
         <!-- Configuration to enable functional tests -->
         <env name="FUNC_ENABLED" value="false" />
         <env name="FUNC_API_URI" value="https://mxm.xtremepush.com/" />
-        <env name="FUNC_API_TOKEN" value="apitoken" />
+        <env name="FUNC_API_USERNAME" value="clientId" />
+        <env name="FUNC_API_PASSWORD" value="clientSecret" />
     </php>
 
     <testsuites>
diff --git a/src/Client.php b/src/Client.php
index 9a716b5..d0856a0 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -64,8 +64,6 @@ class Client implements LoggerAwareInterface
 
     private string $uri = 'https://mxm.xtremepush.com/';
 
-    private readonly string $token;
-
     private readonly string $username;
 
     private readonly string $password;
@@ -90,26 +88,20 @@ class Client implements LoggerAwareInterface
 
     /**
      * @param array{
-     *     token: string, // Required, or username & password
-     *     username: string, // Required, if no token
-     *     password: string, // Required, if no token
+     *     username: string, // Required; API client ID
+     *     password: string, // Required; API client secret
      *     uri: string, // Optional. Default https://mxm.xtremepush.com/
      *     debugLogging: bool, // Optional. Enable logging of request/response. Default false
      * } $config
      */
     public function __construct(array $config)
     {
-        // Must have API token
-        if (!isset($config['token'])) {
-            // Must have user/pass
-            if (!isset($config['username']) || !isset($config['password'])) {
-                throw new Exception\InvalidArgumentException('API config requires token OR username & password');
-            }
-            $this->username = $config['username'];
-            $this->password = $config['password'];
-        } else {
-            $this->token = $config['token'];
+        // Must have user/pass
+        if (!isset($config['username']) || !isset($config['password'])) {
+            throw new Exception\InvalidArgumentException('API config requires username & password');
         }
+        $this->username = $config['username'];
+        $this->password = $config['password'];
 
         if (isset($config['uri'])) {
             $parsed = parse_url($config['uri']);
@@ -153,6 +145,10 @@ private function getClient(): GuzzleClientInterface
 
             $clientConfig = [
                 'base_uri' => $this->uri . 'api/json/',
+                'auth' => [
+                    $this->username,
+                    $this->password,
+                ],
                 'headers' => [
                     'User-Agent' => 'MxmApiClient/' . self::VERSION . ' PHP/' . PHP_VERSION,
                     'Content-Type' => 'application/x-www-form-urlencoded',
@@ -161,15 +157,6 @@ private function getClient(): GuzzleClientInterface
                 'handler' => $stack,
             ];
 
-            if (isset($this->token)) {
-                $clientConfig['headers']['Authorization'] = 'Bearer ' . $this->token;
-            } else {
-                $clientConfig['auth'] = [
-                    $this->username,
-                    $this->password,
-                ];
-            }
-
             if (!isset($this->httpClientFactory)) {
                 $this->httpClient = new GuzzleClient($clientConfig);
             } else {
diff --git a/tests/ClientTest.php b/tests/ClientTest.php
index 4c3dbe0..1cbeb1c 100644
--- a/tests/ClientTest.php
+++ b/tests/ClientTest.php
@@ -19,7 +19,8 @@ class ClientTest extends TestCase
 {
     private array $testConfig = [
         'uri' => 'https://maxemail.example.com/',
-        'token' => 'apitoken',
+        'username' => 'clientId',
+        'password' => 'clientSecret',
     ];
 
     public function testConfigValid(): void
@@ -30,11 +31,16 @@ public function testConfigValid(): void
             $expectedUri = $this->testConfig['uri'] . 'api/json/';
             static::assertSame($expectedUri, $actual['base_uri']);
 
+            $expectedAuth = [
+                $this->testConfig['username'],
+                $this->testConfig['password'],
+            ];
+            static::assertSame($expectedAuth, $actual['auth']);
+
             $expectedHeaders = [
                 'User-Agent' => 'MxmApiClient/' . Client::VERSION . ' PHP/' . PHP_VERSION,
                 'Content-Type' => 'application/x-www-form-urlencoded',
                 'Accept' => 'application/json',
-                'Authorization' => 'Bearer ' . $this->testConfig['token'],
             ];
             static::assertSame($expectedHeaders, $actual['headers']);
 
@@ -50,7 +56,8 @@ public function testConfigValid(): void
     public function testConfigDefaultHost(): void
     {
         $config = [
-            'token' => 'apitoken',
+            'username' => 'clientId',
+            'password' => 'clientSecret',
         ];
 
         $api = new Client($config);
@@ -72,7 +79,8 @@ public function testConfigStripsUriPath(): void
     {
         $config = [
             'uri' => 'https://maxemail.example.com/some/extra/path',
-            'token' => 'apitoken',
+            'username' => 'clientId',
+            'password' => 'clientSecret',
         ];
 
         $api = new Client($config);
@@ -97,7 +105,8 @@ public function testConfigInvalidUri(): void
 
         $config = [
             'uri' => '//',
-            'token' => 'apitoken',
+            'username' => 'clientId',
+            'password' => 'clientSecret',
         ];
 
         new Client($config);
@@ -110,52 +119,20 @@ public function testConfigMissingUriProtocol(): void
 
         $config = [
             'uri' => 'maxemail.example.com',
-            'token' => 'apitoken',
+            'username' => 'clientId',
+            'password' => 'clientSecret',
         ];
 
         new Client($config);
     }
 
-    public function testConfigLegacyAuthentication(): void
-    {
-        $config = [
-            'username' => 'api@user.com',
-            'password' => 'apipass',
-        ];
-
-        $api = new Client($config);
-
-        $factory = function (array $actual) use ($config): GuzzleClient {
-            $expectedAuth = [
-                $config['username'],
-                $config['password'],
-            ];
-            static::assertSame($expectedAuth, $actual['auth']);
-
-            return $this->createMock(GuzzleClient::class);
-        };
-
-        $api->setHttpClientFactory($factory);
-
-        // Get a service, to trigger the HTTP Client factory
-        $api->folder;
-    }
-
-    public function testConfigMissingToken(): void
-    {
-        $this->expectException(Exception\InvalidArgumentException::class);
-        $this->expectExceptionMessage('API config requires token OR username & password');
-
-        new Client([]);
-    }
-
     public function testConfigMissingUsername(): void
     {
         $this->expectException(Exception\InvalidArgumentException::class);
-        $this->expectExceptionMessage('API config requires token OR username & password');
+        $this->expectExceptionMessage('API config requires username & password');
 
         $config = [
-            'password' => 'apipass',
+            'password' => 'clientSecret',
         ];
 
         new Client($config);
@@ -164,39 +141,20 @@ public function testConfigMissingUsername(): void
     public function testConfigMissingPassword(): void
     {
         $this->expectException(Exception\InvalidArgumentException::class);
-        $this->expectExceptionMessage('API config requires token OR username & password');
+        $this->expectExceptionMessage('API config requires username & password');
 
         $config = [
-            'username' => 'api@user.com',
+            'username' => 'clientId',
         ];
 
         new Client($config);
     }
 
-    public function testGetConfigWithToken(): void
+    public function testGetConfig(): void
     {
         $api = new Client($this->testConfig);
 
-        $expected = [
-            'uri' => $this->testConfig['uri'],
-            'username' => null,
-            'password' => null,
-        ];
-
-        static::assertSame($expected, $api->getConfig());
-    }
-
-    public function testGetConfigWithLegacyAuthentication(): void
-    {
-        $config = [
-            'uri' => 'https://maxemail.example.com/',
-            'username' => 'api@user.com',
-            'password' => 'apipass',
-        ];
-
-        $api = new Client($config);
-
-        static::assertSame($config, $api->getConfig());
+        static::assertSame($this->testConfig, $api->getConfig());
     }
 
     public function testSetGetLogger(): void
diff --git a/tests/FunctionalTest.php b/tests/FunctionalTest.php
index ccb23d9..d11fa51 100644
--- a/tests/FunctionalTest.php
+++ b/tests/FunctionalTest.php
@@ -27,7 +27,8 @@ protected function setUp(): void
 
         $config = [
             'uri' => getenv('FUNC_API_URI'),
-            'token' => getenv('FUNC_API_TOKEN'),
+            'username' => getenv('FUNC_API_USERNAME'),
+            'password' => getenv('FUNC_API_PASSWORD'),
         ];
         $this->client = new Client($config);
     }