diff --git a/spec/custom_matchers/equal_namespace_definitions_spec.rb b/spec/custom_matchers/equal_namespace_definitions_spec.rb
new file mode 100644
index 0000000..02b51aa
--- /dev/null
+++ b/spec/custom_matchers/equal_namespace_definitions_spec.rb
@@ -0,0 +1,98 @@
+# frozen_string_literal: true
+
+RSpec.describe 'equal_namespace_definitions matcher' do
+  let(:xml) { Examples::Example2.xml }
+  let(:xml2) { Examples::Example2.xml }
+
+  it 'successfully compares two similar xml strings' do
+    expect(xml).to have_equal_namespace_definitions_as xml2
+  end
+
+  context 'when one of the classes is not a string' do
+    let(:string) { 'hello' }
+    let(:integer) { 1234 }
+
+    it 'fails' do
+      expect(string).not_to have_equal_namespace_definitions_as integer
+    end
+
+    it 'provides a useful error message' do
+      expect { expect(string).to have_equal_namespace_definitions_as integer }
+        .to raise_error(RSpec::Expectations::ExpectationNotMetError, /does not have equal namespaces as/)
+    end
+  end
+
+  context 'when one of the strings does not contain valid xml' do
+    let(:xml2) { 'foobar' }
+
+    it 'fails' do
+      expect(xml).not_to have_equal_namespace_definitions_as xml2
+    end
+
+    it 'provides a useful error message' do
+      expect { expect(xml).to have_equal_namespace_definitions_as xml2 }
+        .to raise_error(RSpec::Expectations::ExpectationNotMetError, /does not have equal namespaces as/)
+    end
+  end
+
+  context 'when one xml string is different' do
+    let(:xml2) { Examples::Example3.json }
+
+    it 'fails the comparison' do
+      expect(xml).not_to have_equal_namespace_definitions_as xml2
+    end
+
+    it 'provides a useful error message' do
+      expect { expect(xml).to have_equal_namespace_definitions_as xml2 }
+        .to raise_error(RSpec::Expectations::ExpectationNotMetError, /does not have equal namespaces as/)
+    end
+  end
+
+  context 'with namespaces' do
+    let(:xml) do
+      <<-XML
+        <alice xmlns="http://some-namespace" xmlns:charlie="http://some-other-namespace">
+          <bob>david</bob>
+          <charlie:edgar>frank</charlie:edgar>
+        </alice>
+      XML
+    end
+
+    context 'with unequal namespace definitions' do
+      let(:xml2) do
+        <<~XML
+          <alice xmlns:charlie="http://some-other-namespace" xmlns="http://some-namespace">
+            <bob>david</bob>
+            <charlie:edgar xmlns:charlie="http://some-other-namespace" xmlns="http://some-namespace">
+              frank
+            </charlie:edgar>
+          </alice>
+        XML
+      end
+
+      it 'fails the comparison' do
+        expect(xml).not_to have_equal_namespace_definitions_as xml2
+      end
+
+      it 'provides a useful error message' do
+        expect { expect(xml).to have_equal_namespace_definitions_as xml2 }
+          .to raise_error(RSpec::Expectations::ExpectationNotMetError, /does not have equal namespaces as/)
+      end
+    end
+
+    context 'with similar namespace definitions' do
+      let(:xml2) do
+        <<~XML
+          <alice xmlns:charlie="http://some-other-namespace" xmlns="http://some-namespace">
+            <bob>david</bob>
+            <charlie:edgar>frank</charlie:edgar>
+          </alice>
+        XML
+      end
+
+      it 'successfully compares two similar xml strings' do
+        expect(xml).to have_equal_namespace_definitions_as xml2
+      end
+    end
+  end
+end
diff --git a/spec/dachsfisch/bidirectional_converter_spec.rb b/spec/dachsfisch/bidirectional_converter_spec.rb
new file mode 100644
index 0000000..f32c789
--- /dev/null
+++ b/spec/dachsfisch/bidirectional_converter_spec.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+RSpec.describe 'BidirectionalConverter' do
+  let(:xml2_json) { Dachsfisch::XML2JSONConverter }
+  let(:json2_xml) { Dachsfisch::JSON2XMLConverter }
+
+  context 'with valid XML' do
+    describe '#perform' do
+      subject { json2_xml.perform json: }
+
+      let(:json) { xml2_json.perform xml: }
+
+      Examples.each :json2xml do |example|
+        context "with #{example.name}" do
+          let(:xml) { example.xml }
+
+          it { is_expected.to be_equivalent_to(xml) }
+          it { is_expected.to have_equal_namespace_definitions_as(xml) }
+        end
+      end
+    end
+  end
+
+  context 'with valid JSON' do
+    describe '#perform' do
+      subject { xml2_json.perform xml: }
+
+      let(:xml) { json2_xml.perform json: }
+
+      Examples.each :xml2json do |example|
+        context "with #{example.name}" do
+          let(:json) { example.json }
+
+          it { is_expected.to be_an_equal_json_as(json) }
+        end
+      end
+    end
+  end
+end
diff --git a/spec/support/expectations/equal_namespace_definitions.rb b/spec/support/expectations/equal_namespace_definitions.rb
new file mode 100644
index 0000000..8f33794
--- /dev/null
+++ b/spec/support/expectations/equal_namespace_definitions.rb
@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+require 'rspec/expectations'
+
+RSpec::Matchers.define :have_equal_namespace_definitions_as do |expected|
+  attr_reader :actual, :expected
+
+  match do |actual|
+    return false unless actual.is_a?(String) && expected.is_a?(String)
+
+    expected_xml = parse_fragment(expected)
+    @expected = expected_xml.to_xml
+    actual_xml = parse_fragment(actual)
+    @actual = actual_xml.to_xml
+
+    return false if expected_xml.errors.length.positive? || actual_xml.errors.length.positive?
+    return false unless EquivalentXml.equivalent?(expected_xml, actual_xml)
+
+    return compare_namespaces(actual_xml.children.first, expected_xml.children.first)
+  end
+
+  failure_message do |actual|
+    "#{@actual || actual} does not have equal namespaces as \n#{@expected || expected}."
+  end
+
+  diffable
+
+  private
+
+  def compare_namespaces(actual, expected)
+    return false unless same_namespace_definitions?(actual, expected)
+
+    actual.children.each_with_index.all? do |actual_child, index|
+      expected_child = expected.children[index]
+      compare_namespaces(actual_child, expected_child)
+    end
+  end
+
+  def same_namespace_definitions?(actual, expected)
+    return true if actual.nil? && expected.nil?
+    return false if actual.nil? || expected.nil?
+
+    actual_namespaces = namespaces(actual)
+    expected_namespaces = namespaces(expected)
+    actual_namespaces == expected_namespaces
+  end
+
+  def namespaces(node)
+    node.namespace_definitions.map {|namespace| namespace.deconstruct_keys(%i[prefix href]) }.sort_by {|ns| ns[:prefix].to_s }
+  end
+
+  def parse_fragment(xml)
+    # This is a workaround for an unintended behavior in Nokogiri's XML::DocumentFragment.parse method.
+    # Originally, the method de-duplicates the namespace definitions of all nodes in the fragment, if possible.
+    # However, for this test, we want to compare the namespace definitions of the actual and expected XML.
+    # Therefore, we parse the XML with a root node, and compare the resulting document.
+    Nokogiri::XML::Document.parse("<root>#{xml}</root>", &:noblanks)
+  end
+end