core/modules/serialization/src/Encoder/JsonEncoder.php              | 4 ++++
 core/modules/serialization/src/Encoder/XmlEncoder.php               | 4 ++++
 .../serialization/src/RegisterSerializationClassesCompilerPass.php  | 6 ++++++
 3 files changed, 14 insertions(+)

diff --git a/core/modules/serialization/src/Encoder/JsonEncoder.php b/core/modules/serialization/src/Encoder/JsonEncoder.php
index ad085ac..964c295 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 9db6ecd..53177b0 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 {
 
diff --git a/core/modules/serialization/src/RegisterSerializationClassesCompilerPass.php b/core/modules/serialization/src/RegisterSerializationClassesCompilerPass.php
index 10474d1..3979e01 100644
--- a/core/modules/serialization/src/RegisterSerializationClassesCompilerPass.php
+++ b/core/modules/serialization/src/RegisterSerializationClassesCompilerPass.php
@@ -23,6 +23,9 @@ public function process(ContainerBuilder $container) {
 
     // Retrieve registered Normalizers and Encoders from the container.
     foreach ($container->findTaggedServiceIds('normalizer') as $id => $attributes) {
+      // The 'serializer' service is the public API: mark normalizers private.
+      $container->getDefinition($id)->setPublic(FALSE);
+
       // If there is a BC key present, pass this to determine if the normalizer
       // should be skipped.
       if (isset($attributes[0]['bc']) && $this->normalizerBcSettingIsEnabled($attributes[0]['bc'], $attributes[0]['bc_config_name'])) {
@@ -33,6 +36,9 @@ public function process(ContainerBuilder $container) {
       $normalizers[$priority][] = new Reference($id);
     }
     foreach ($container->findTaggedServiceIds('encoder') as $id => $attributes) {
+      // The 'serializer' service is the public API: mark encoders private.
+      $container->getDefinition($id)->setPublic(FALSE);
+
       $priority = isset($attributes[0]['priority']) ? $attributes[0]['priority'] : 0;
       $encoders[$priority][] = new Reference($id);
     }