diff --git a/core/modules/serialization/src/Encoder/JsonEncoder.php b/core/modules/serialization/src/Encoder/JsonEncoder.php
index ad085ac00aa88c9b7bfb213de821cd212370d64e..964c2951f60b112c681981dc806f5d17f49d1a43 100644
--- a/core/modules/serialization/src/Encoder/JsonEncoder.php
+++ b/core/modules/serialization/src/Encoder/JsonEncoder.php
@@ -10,6 +10,10 @@
 
 /**
  * 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 9db6ecde0123c919783aacbfba58f910a87777f0..53177b06cbaf7f598e06ec993b24fdfa54a55224 100644
--- a/core/modules/serialization/src/Encoder/XmlEncoder.php
+++ b/core/modules/serialization/src/Encoder/XmlEncoder.php
@@ -12,6 +12,10 @@
  *
  * 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 {