diff --git a/src/Client/Decorator/OAuth2/ClientCredentials.php b/src/Client/Decorator/OAuth2/ClientCredentials.php
index fe9684e..9a51066 100644
--- a/src/Client/Decorator/OAuth2/ClientCredentials.php
+++ b/src/Client/Decorator/OAuth2/ClientCredentials.php
@@ -37,6 +37,16 @@ private function __construct(string $clientID, string $clientSecret, string $sco
         $this->scope = mb_convert_encoding($scope, 'UTF-8', 'ISO-8859-1');
     }
 
+    public function getClientId(): string
+    {
+        return $this->clientId;
+    }
+
+    public function getClientSecret(): string
+    {
+        return $this->clientSecret;
+    }
+
     /**
      * Named constructor to create an instance based on the given credentials
      *
@@ -52,14 +62,17 @@ public static function forHeaderAuthorization(string $clientId, string $clientSe
     /**
      * Creates the required key => value pairs for the Access Token Request
      */
-    public function toArray(): array
+    public function toArray(bool $includeClientCredentials = true): array
     {
         $requestData = [
             'grant_type' => 'client_credentials',
-            'client_id' => $this->clientId,
-            'client_secret' => $this->clientSecret,
         ];
 
+        if ($includeClientCredentials) {
+            $requestData['client_id'] = $this->clientId;
+            $requestData['client_secret'] = $this->clientSecret;
+        }
+
         if (!empty($this->scope)) {
             $requestData['scope'] = $this->scope;
         }
diff --git a/src/Client/Decorator/OAuth2/ClientCredentialsDecorator.php b/src/Client/Decorator/OAuth2/ClientCredentialsDecorator.php
index 017ed2c..20f23f5 100644
--- a/src/Client/Decorator/OAuth2/ClientCredentialsDecorator.php
+++ b/src/Client/Decorator/OAuth2/ClientCredentialsDecorator.php
@@ -25,7 +25,9 @@
 use Artemeon\HttpClient\Http\Body\Body;
 use Artemeon\HttpClient\Http\Body\Encoder\FormUrlEncoder;
 use Artemeon\HttpClient\Http\Header\Fields\Authorization;
+use Artemeon\HttpClient\Http\Header\Header;
 use Artemeon\HttpClient\Http\Header\HeaderField;
+use Artemeon\HttpClient\Http\Header\Headers;
 use Artemeon\HttpClient\Http\MediaType;
 use Artemeon\HttpClient\Http\Request;
 use Artemeon\HttpClient\Http\Response;
@@ -66,23 +68,34 @@ public function __construct(
      * @param Uri $uri The Uri object
      * @param HttpClient $httpClient The http client to decorate
      * @param AccessTokenCache|null $accessTokenCache AccessTokenCache implementation
+     * @param bool $strictRfc Strictly follow the OAuth2 spec and send the client credentials only in the Authorization header
      *
      * @throws InvalidArgumentException
      * @throws RuntimeException
+     *
+     * @see https://datatracker.ietf.org/doc/html/rfc6749#section-4.4.2
      */
     public static function fromClientCredentials(
         ClientCredentials $clientCredentials,
         Uri $uri,
         HttpClient $httpClient,
-        AccessTokenCache $accessTokenCache = null
+        AccessTokenCache $accessTokenCache = null,
+        bool $strictRfc = false
     ): self {
         // Ensure default cache strategy
         if ($accessTokenCache === null) {
             $accessTokenCache = new InMemoryAccessTokenCache();
         }
 
-        $body = Body::fromEncoder(FormUrlEncoder::fromArray($clientCredentials->toArray()));
-        $accessTokenRequest = Request::forPost($uri, $body);
+        $headers = Headers::create();
+        if ($strictRfc) {
+            $headers->add(Header::fromField(Authorization::forAuthBasic($clientCredentials->getClientId(), $clientCredentials->getClientSecret())));
+            $body = Body::fromEncoder(FormUrlEncoder::fromArray($clientCredentials->toArray(false)));
+        } else {
+            $body = Body::fromEncoder(FormUrlEncoder::fromArray($clientCredentials->toArray()));
+        }
+
+        $accessTokenRequest = Request::forPost($uri, $body, $headers);
 
         return new self($httpClient, $accessTokenRequest, $accessTokenCache);
     }