diff --git a/src/Client/Decorator/OAuth2/ClientCredentials.php b/src/Client/Decorator/OAuth2/ClientCredentials.php
index e616620..fe9684e 100644
--- a/src/Client/Decorator/OAuth2/ClientCredentials.php
+++ b/src/Client/Decorator/OAuth2/ClientCredentials.php
@@ -13,8 +13,6 @@
 
 namespace Artemeon\HttpClient\Client\Decorator\OAuth2;
 
-use function utf8_encode;
-
 /**
  * Class to generate client credentials for OAuth2 Access Token Request's
  */
@@ -34,9 +32,9 @@ class ClientCredentials
     private function __construct(string $clientID, string $clientSecret, string $scope = '')
     {
         // According to the rfc https://tools.ietf.org/html/rfc6749#page-43 encoding must UTF-8
-        $this->clientId = utf8_encode($clientID);
-        $this->clientSecret = utf8_encode($clientSecret);
-        $this->scope = utf8_encode($scope);
+        $this->clientId = mb_convert_encoding($clientID, 'UTF-8', 'ISO-8859-1');
+        $this->clientSecret = mb_convert_encoding($clientSecret, 'UTF-8', 'ISO-8859-1');
+        $this->scope = mb_convert_encoding($scope, 'UTF-8', 'ISO-8859-1');
     }
 
     /**
diff --git a/tests/System/test_post_multipart.php b/tests/System/test_post_multipart.php
index 23c1d74..3efcb97 100644
--- a/tests/System/test_post_multipart.php
+++ b/tests/System/test_post_multipart.php
@@ -29,7 +29,7 @@
         Body::fromEncoder(
             MultipartFormDataEncoder::create()
                 ->addFieldPart('user', 'John.Doe')
-                ->addFieldPart('password', utf8_encode('geheim'))
+                ->addFieldPart('password', mb_convert_encoding('geheim', 'UTF-8', 'ISO-8859-1'))
                 ->addFilePart('user_image', 'header_logo.png', Stream::fromFile('../Fixtures/reader/header_logo.png'))
         )
     );