diff --git a/core/modules/serialization/src/Encoder/JsonEncoder.php b/core/modules/serialization/src/Encoder/JsonEncoder.php
index ad085ac00aa..964c2951f60 100644
--- a/core/modules/serialization/src/Encoder/JsonEncoder.php
+++ b/core/modules/serialization/src/Encoder/JsonEncoder.php
@@ -10,6 +10,10 @@ use Symfony\Component\Serializer\Encoder\JsonEncoder as BaseJsonEncoder;
 
 /**
  * Adds 'ajax to the supported content types of the JSON encoder'
+ *
+ * @internal
+ *   This encoder should not be used directly. Rather, use the `serializer`
+ *   service.
  */
 class JsonEncoder extends BaseJsonEncoder implements EncoderInterface, DecoderInterface {
 
diff --git a/core/modules/serialization/src/Encoder/XmlEncoder.php b/core/modules/serialization/src/Encoder/XmlEncoder.php
index 9db6ecde012..53177b06cba 100644
--- a/core/modules/serialization/src/Encoder/XmlEncoder.php
+++ b/core/modules/serialization/src/Encoder/XmlEncoder.php
@@ -12,6 +12,10 @@ use Symfony\Component\Serializer\Encoder\XmlEncoder as BaseXmlEncoder;
  *
  * This acts as a wrapper class for Symfony's XmlEncoder so that it is not
  * implementing NormalizationAwareInterface, and can be normalized externally.
+ *
+ * @internal
+ *   This encoder should not be used directly. Rather, use the `serializer`
+ *   service.
  */
 class XmlEncoder extends SerializerAwareEncoder implements EncoderInterface, DecoderInterface {