diff --git a/spec/omml/wordprocessing_in_math_spec.rb b/spec/omml/wordprocessing_in_math_spec.rb
index e3918bf..34a3916 100644
--- a/spec/omml/wordprocessing_in_math_spec.rb
+++ b/spec/omml/wordprocessing_in_math_spec.rb
@@ -36,7 +36,7 @@ def run_of(xml)
 
     it "round-trips through serialize and re-parse" do
       parsed = Omml.parse(wrap('<w:br w:type="page"/>'))
-      serialized = parsed.to_xml(prefix: "m")
+      serialized = parsed.to_xml(use_prefix: true)
       reparsed = Omml.parse(serialized)
 
       expect(reparsed.r.first.br).to be_a(Omml::Models::CTBr)
@@ -94,7 +94,7 @@ def run_of(xml)
     it "round-trips a complex run with multiple wordprocessing children" do
       xml = wrap("<w:cr/><w:t>x</w:t><w:tab/><m:t>y</m:t>")
       parsed = Omml.parse(xml)
-      serialized = parsed.to_xml(prefix: "m")
+      serialized = parsed.to_xml(use_prefix: true)
       reparsed = Omml.parse(serialized)
 
       r = reparsed.r.first
@@ -105,16 +105,24 @@ def run_of(xml)
   end
 
   describe "programmatic construction" do
-    it "builds a CTR with CTBr and serializes to wordprocessing namespace" do
+    it "builds a CTR with CTBr and serializes br with w: prefix" do
       br = Omml::Models::CTBr.new(type: "page")
       r = Omml::Models::CTR.new(br: br)
 
-      # The TYPE namespace determines the element's namespace URI.
-      # CTBr is in wordprocessing, so the element URI must be wordprocessing
-      # regardless of the prefix the serializer chooses to emit.
-      xml = r.to_xml(prefix: "m")
-      expect(xml).to include("http://schemas.openxmlformats.org/wordprocessingml/2006/main")
-      expect(xml).to include("br")
+      # `use_prefix: true` makes the serializer honor each namespace's
+      # `prefix_default` (m for math, w for wordprocessing). Required for
+      # Word compatibility — Word rejects math elements in default xmlns
+      # form; it requires the `m:` prefix on every element.
+      xml = r.to_xml(use_prefix: true)
+      doc = Omml::Configuration.xml_adapter.parse(xml).root
+
+      expect(doc.namespace.uri)
+        .to eq("http://schemas.openxmlformats.org/officeDocument/2006/math")
+      br_element = doc.children.find(&:element?)
+      expect(br_element.unprefixed_name).to eq("br")
+      expect(br_element.namespace.uri)
+        .to eq("http://schemas.openxmlformats.org/wordprocessingml/2006/main")
+      expect(br_element.namespace.prefix).to eq("w")
     end
   end
 end
diff --git a/spec/omml_fixture_round_trip_spec.rb b/spec/omml_fixture_round_trip_spec.rb
index a01b115..f174971 100644
--- a/spec/omml_fixture_round_trip_spec.rb
+++ b/spec/omml_fixture_round_trip_spec.rb
@@ -166,7 +166,7 @@ def expected_root_class(xml)
   def expect_fixture_to_round_trip(fixture_path)
     xml = File.read(fixture_path)
     parsed = Omml.parse(xml)
-    serialized = parsed.to_xml(prefix: "m")
+    serialized = parsed.to_xml(use_prefix: true)
     diff = xml_difference(xml, serialized)
 
     expect(parsed).to be_a(expected_root_class(xml))
diff --git a/spec/omml_spec.rb b/spec/omml_spec.rb
index 01fd510..aebd441 100644
--- a/spec/omml_spec.rb
+++ b/spec/omml_spec.rb
@@ -87,8 +87,8 @@ def wordprocessing_run_properties(parsed)
 
     expect(parsed).to be_a(Omml::Models::OMath)
     expect(parsed.r.first.t.content).to eq("x")
-    expect(parsed.to_xml(prefix: "m")).to include("<m:oMath")
-    expect(parsed.to_xml(prefix: "m")).to include("<m:t>x</m:t>")
+    expect(parsed.to_xml(use_prefix: true)).to include("<m:oMath")
+    expect(parsed.to_xml(use_prefix: true)).to include("<m:t>x</m:t>")
   end
 
   it "parses roots with leading declarations and comments" do
@@ -101,7 +101,7 @@ def wordprocessing_run_properties(parsed)
   it "preserves wordprocessing on-off run properties under ctrlPr" do
     parsed = described_class.parse(o_math_para_with_wordprocessing_flags_xml)
     r_pr = wordprocessing_run_properties(parsed)
-    serialized = parsed.to_xml(prefix: "m")
+    serialized = parsed.to_xml(use_prefix: true)
     reparsed = described_class.parse(serialized)
     reparsed_r_pr = wordprocessing_run_properties(reparsed)