diff --git a/lib/active_model/serializer/attributes.rb b/lib/active_model/serializer/attributes.rb
index f46b0f48..81f6e49a 100644
--- a/lib/active_model/serializer/attributes.rb
+++ b/lib/active_model/serializer/attributes.rb
@@ -1,6 +1,7 @@
 module ActiveModel
   class Serializer
     module Attributes
+      # @api private
       class Attribute
         delegate :call, to: :reader
 
@@ -19,12 +20,14 @@ module ActiveModel
           end
         end
       end
+      # @api private
       class AttributeReader < Attribute
         def initialize(name)
           super(name)
           @reader = ->(instance) { instance.read_attribute_for_serialization(name) }
         end
       end
+      # @api private
       class AttributeBlock < Attribute
         def initialize(name, block)
           super(name)
diff --git a/lib/active_model/serializer/reflection.rb b/lib/active_model/serializer/reflection.rb
index 472b2991..c027d96e 100644
--- a/lib/active_model/serializer/reflection.rb
+++ b/lib/active_model/serializer/reflection.rb
@@ -35,10 +35,12 @@ module ActiveModel
         @reader = self.class.build_reader(name, block)
       end
 
+      # @api private
       def value(instance)
         call(instance)
       end
 
+      # @api private
       def self.build_reader(name, block)
         if block
           ->(instance) { instance.read_attribute_for_serialization(name).instance_eval(&block) }