diff --git a/lib/active_model_serializers/adapter/json_api/deserialization.rb b/lib/active_model_serializers/adapter/json_api/deserialization.rb
index f5244355..ff1d3785 100644
--- a/lib/active_model_serializers/adapter/json_api/deserialization.rb
+++ b/lib/active_model_serializers/adapter/json_api/deserialization.rb
@@ -155,7 +155,7 @@ module ActiveModelSerializers
 
         # @api private
         def parse_attributes(attributes, options)
-          attributes
+          transform_keys(attributes, options)
             .map { |(k, v)| { field_key(k, options) => v } }
             .reduce({}, :merge)
         end
@@ -182,23 +182,29 @@ module ActiveModelSerializers
           prefix_key = field_key(assoc_name, options).to_s.singularize
           hash =
             if assoc_data.is_a?(Array)
-              { "#{prefix_key}_ids".to_sym => assoc_data.map { |ri| ri['id'] } }
+              { "#{prefix_key}_ids".to_sym => assoc_data.map { |ri| ri[:id] } }
             else
-              { "#{prefix_key}_id".to_sym => assoc_data ? assoc_data['id'] : nil }
+              { "#{prefix_key}_id".to_sym => assoc_data && assoc_data.is_a?(Hash) ? assoc_data[:id] : nil }
             end
 
           polymorphic = (options[:polymorphic] || []).include?(assoc_name.to_sym)
-          hash["#{prefix_key}_type".to_sym] = assoc_data['type'] if polymorphic
+          hash["#{prefix_key}_type".to_sym] = assoc_data[:type] if polymorphic
 
           hash
         end
 
         # @api private
         def parse_relationships(relationships, options)
-          relationships
-            .map { |(k, v)| parse_relationship(k, v['data'], options) }
+          transform_keys(relationships, options)
+            .map { |(k, v)| parse_relationship(k, v[:data], options) }
             .reduce({}, :merge)
         end
+
+        # @api private
+        def transform_keys(hash, options)
+          transform = options[:key_transform] || :underscore
+          KeyTransform.send(transform, hash)
+        end
       end
     end
   end
diff --git a/lib/active_model_serializers/key_transform.rb b/lib/active_model_serializers/key_transform.rb
index 03cf9cef..cf120547 100644
--- a/lib/active_model_serializers/key_transform.rb
+++ b/lib/active_model_serializers/key_transform.rb
@@ -32,6 +32,16 @@ module ActiveModelSerializers
       hash.deep_transform_keys! { |key| key.to_s.dasherize.to_sym }
     end
 
+    # Transforms keys to underscore.
+    # This is the default case for deserialization in the JsonApi adapter.
+    #
+    # @example:
+    #    "some-key" => "some_key",
+    # @see {https://github.com/rails/rails/blob/master/activesupport/lib/active_support/inflector/methods.rb#L89-L98 ActiveSupport::Inflector.underscore}
+    def underscore(hash)
+      hash.deep_transform_keys! { |key| key.to_s.underscore.to_sym }
+    end
+
     # Returns the hash unaltered
     def unaltered(hash)
       hash
diff --git a/test/action_controller/json_api/deserialization_test.rb b/test/action_controller/json_api/deserialization_test.rb
index 8a57d594..0528dc55 100644
--- a/test/action_controller/json_api/deserialization_test.rb
+++ b/test/action_controller/json_api/deserialization_test.rb
@@ -20,7 +20,10 @@ module ActionController
               'id' => 'zorglub',
               'attributes' => {
                 'title' => 'Ember Hamster',
-                'src' => 'http://example.com/images/productivity.png'
+                'src' => 'http://example.com/images/productivity.png',
+                'image-width' => '200',
+                'imageHeight' => '200',
+                'ImageSize' => '1024'
               },
               'relationships' => {
                 'author' => {
@@ -34,6 +37,12 @@ module ActionController
                     { 'type' => 'comments', 'id' => '1' },
                     { 'type' => 'comments', 'id' => '2' }
                   ]
+                },
+                'related-images' => {
+                  'data' => [
+                    { 'type' => 'image', 'id' => '7' },
+                    { 'type' => 'image', 'id' => '8' }
+                  ]
                 }
               }
             }
@@ -46,9 +55,13 @@ module ActionController
             'id' => 'zorglub',
             'title' => 'Ember Hamster',
             'src' => 'http://example.com/images/productivity.png',
+            'image_width' => '200',
+            'image_height' => '200',
+            'image_size' => '1024',
             'author_id' => nil,
             'photographer_id' => '9',
-            'comment_ids' => %w(1 2)
+            'comment_ids' => %w(1 2),
+            'related_image_ids' => %w(7 8)
           }
 
           assert_equal(expected, response)