diff --git a/test/test_serialization_encoding.rb b/test/test_serialization_encoding.rb
new file mode 100644
index 00000000000..c8f6386ce23
--- /dev/null
+++ b/test/test_serialization_encoding.rb
@@ -0,0 +1,198 @@
+# coding: utf-8
+# frozen_string_literal: true
+
+require "helper"
+
+class TestSerializationEncoding < Nokogiri::TestCase
+  def round_trip_through_file
+    Tempfile.create do |io|
+      yield io
+      io.rewind
+      io.read
+    end
+  end
+
+  describe "serialization encoding" do
+    matrix = [
+      {
+        klass: Nokogiri::XML::Document,
+        documents: [
+          { encoding: Encoding::UTF_8, path: ADDRESS_XML_FILE },
+          { encoding: Encoding::Shift_JIS, path: SHIFT_JIS_XML },
+        ],
+      },
+      {
+        klass: Nokogiri::HTML4::Document,
+        documents: [
+          { encoding: Encoding::UTF_8, path: HTML_FILE },
+          { encoding: Encoding::Shift_JIS, path: SHIFT_JIS_HTML },
+        ],
+      },
+    ]
+    if Nokogiri.uses_gumbo?
+      matrix << {
+        klass: Nokogiri::HTML5::Document,
+        documents: [
+          { encoding: Encoding::UTF_8, path: HTML_FILE },
+          { encoding: Encoding::Shift_JIS, path: SHIFT_JIS_HTML },
+        ],
+      }
+    end
+
+    matrix.each do |matrix_entry|
+      describe matrix_entry[:klass] do
+        let(:klass) { matrix_entry[:klass] }
+        matrix_entry[:documents].each do |document|
+          describe document[:encoding] do
+            it "serializes with the expected encoding" do
+              doc = klass.parse(
+                File.read(
+                  document[:path],
+                  encoding: document[:encoding],
+                ),
+              )
+
+              expected_default_encoding =
+                if defined?(Nokogiri::HTML5::Document) && klass == Nokogiri::HTML5::Document
+                  Encoding::UTF_8 # FIXME: see #2801, this should be document[:encoding]
+                else
+                  document[:encoding]
+                end
+
+              assert_equal(expected_default_encoding, doc.to_s.encoding)
+
+              assert_equal(expected_default_encoding, doc.to_xml.encoding)
+              assert_equal(Encoding::UTF_8, doc.to_xml(encoding: "UTF-8").encoding)
+              assert_equal(Encoding::Shift_JIS, doc.to_xml(encoding: "SHIFT_JIS").encoding)
+
+              assert_equal(expected_default_encoding, doc.to_xhtml.encoding)
+              assert_equal(Encoding::UTF_8, doc.to_xhtml(encoding: "UTF-8").encoding)
+              assert_equal(Encoding::Shift_JIS, doc.to_xhtml(encoding: "SHIFT_JIS").encoding)
+
+              assert_equal(expected_default_encoding, doc.to_html.encoding)
+              assert_equal(Encoding::UTF_8, doc.to_html(encoding: "UTF-8").encoding)
+              assert_equal(Encoding::Shift_JIS, doc.to_html(encoding: "SHIFT_JIS").encoding)
+
+              assert_equal(expected_default_encoding, doc.serialize.encoding)
+              assert_equal(Encoding::UTF_8, doc.serialize(encoding: "UTF-8").encoding)
+              assert_equal(Encoding::Shift_JIS, doc.serialize(encoding: "SHIFT_JIS").encoding)
+
+              assert_equal(
+                doc.serialize.bytes,
+                round_trip_through_file { |io| doc.write_to(io) }.bytes,
+              )
+              assert_equal(
+                doc.serialize(encoding: "UTF-8").bytes,
+                round_trip_through_file { |io| doc.write_to(io, encoding: "UTF-8") }.bytes,
+              )
+              assert_equal(
+                doc.serialize(encoding: "SHIFT_JIS").bytes,
+                round_trip_through_file { |io| doc.write_to(io, encoding: "SHIFT_JIS") }.bytes,
+              )
+            end
+          end
+        end
+      end
+    end
+
+    # describe "HTML4" do
+    #   matrix_documents = [
+    #     {encoding: Encoding::UTF_8, path: HTML_FILE},
+    #     {encoding: Encoding::Shift_JIS, path: SHIFT_JIS_HTML},
+    #   ]
+
+    #   matrix_documents.each do |matrix_document|
+    #     describe matrix_document[:encoding] do
+    #       it "serializes with the expected encoding" do
+    #         doc = Nokogiri::HTML4::Document.parse(
+    #           File.read(
+    #             matrix_document[:path],
+    #             encoding: matrix_document[:encoding],
+    #           )
+    #         )
+
+    #         assert_equal(matrix_document[:encoding], doc.to_s.encoding)
+
+    #         assert_equal(matrix_document[:encoding], doc.to_xml.encoding)
+    #         assert_equal(Encoding::UTF_8, doc.to_xml(encoding: "UTF-8").encoding)
+    #         assert_equal(Encoding::Shift_JIS, doc.to_xml(encoding: "SHIFT_JIS").encoding)
+
+    #         assert_equal(matrix_document[:encoding], doc.to_xhtml.encoding)
+    #         assert_equal(Encoding::UTF_8, doc.to_xhtml(encoding: "UTF-8").encoding)
+    #         assert_equal(Encoding::Shift_JIS, doc.to_xhtml(encoding: "SHIFT_JIS").encoding)
+
+    #         assert_equal(matrix_document[:encoding], doc.to_html.encoding)
+    #         assert_equal(Encoding::UTF_8, doc.to_html(encoding: "UTF-8").encoding)
+    #         assert_equal(Encoding::Shift_JIS, doc.to_html(encoding: "SHIFT_JIS").encoding)
+
+    #         assert_equal(matrix_document[:encoding], doc.serialize.encoding)
+    #         assert_equal(Encoding::UTF_8, doc.serialize(encoding: "UTF-8").encoding)
+    #         assert_equal(Encoding::Shift_JIS, doc.serialize(encoding: "SHIFT_JIS").encoding)
+
+    #         payload = round_trip_through_file { |io| doc.write_to(io) }
+    #         assert_equal(doc.serialize.bytes, payload.bytes)
+
+    #         payload = round_trip_through_file { |io| doc.write_to(io, encoding: "UTF-8") }
+    #         assert_equal(doc.serialize(encoding: "UTF-8").bytes, payload.bytes)
+
+    #         payload = round_trip_through_file { |io| doc.write_to(io, encoding: "SHIFT_JIS") }
+    #         assert_equal(doc.serialize(encoding: "SHIFT_JIS").bytes, payload.bytes)
+    #       end
+    #     end
+    #   end
+    # end
+
+    # describe "HTML5" do
+    #   matrix_documents = [
+    #     {encoding: Encoding::UTF_8, path: HTML_FILE},
+    #     {encoding: Encoding::Shift_JIS, path: SHIFT_JIS_HTML},
+    #   ]
+
+    #   matrix_documents.each do |matrix_document|
+    #     describe matrix_document[:encoding] do
+    #       it "serializes with the expected encoding" do
+    #         doc = Nokogiri::HTML5::Document.parse(
+    #           File.read(
+    #             matrix_document[:path],
+    #             encoding: matrix_document[:encoding],
+    #           )
+    #         )
+
+    #         expected_default_encoding = Encoding::UTF_8 # FIXME: see #2801, this should be matrix_document[:encoding]
+
+    #         assert_equal(expected_default_encoding, doc.to_s.encoding)
+
+    #         assert_equal(expected_default_encoding, doc.to_xml.encoding)
+    #         assert_equal(Encoding::UTF_8, doc.to_xml(encoding: "UTF-8").encoding)
+    #         assert_equal(Encoding::Shift_JIS, doc.to_xml(encoding: "SHIFT_JIS").encoding)
+
+    #         assert_equal(expected_default_encoding, doc.to_xhtml.encoding)
+    #         assert_equal(Encoding::UTF_8, doc.to_xhtml(encoding: "UTF-8").encoding)
+    #         assert_equal(Encoding::Shift_JIS, doc.to_xhtml(encoding: "SHIFT_JIS").encoding)
+
+    #         assert_equal(expected_default_encoding, doc.to_html.encoding)
+    #         assert_equal(Encoding::UTF_8, doc.to_html(encoding: "UTF-8").encoding)
+    #         assert_equal(Encoding::Shift_JIS, doc.to_html(encoding: "SHIFT_JIS").encoding)
+
+    #         assert_equal(expected_default_encoding, doc.serialize.encoding)
+    #         assert_equal(Encoding::UTF_8, doc.serialize(encoding: "UTF-8").encoding)
+    #         assert_equal(Encoding::Shift_JIS, doc.serialize(encoding: "SHIFT_JIS").encoding)
+
+    #         assert_equal(
+    #           doc.serialize.bytes,
+    #           round_trip_through_file { |io| doc.write_to(io) }.bytes,
+    #         )
+    #         assert_equal(
+    #           doc.serialize(encoding: "UTF-8").bytes,
+    #           round_trip_through_file { |io| doc.write_to(io, encoding: "UTF-8") }.bytes,
+    #         )
+    #         assert_equal(
+    #           doc.serialize(encoding: "SHIFT_JIS").bytes,
+    #           round_trip_through_file { |io| doc.write_to(io, encoding: "SHIFT_JIS") }.bytes,
+    #         )
+    #       end
+    #     end
+    #   end
+    # end if Nokogiri.uses_gumbo?
+  end
+end