diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 3f6600e..881b1fd 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -4,6 +4,12 @@ Release History
 dev
 ---
 
+**API Changes (Backward Compatible)**
+
+- GoAwayFrame and WindowUpdateFrame now correctly mask off the reserved bit during
+  parsing and serialization of stream IDs and window increments, as per RFC 9113,
+  Sections 6.8 and 6.9.
+
 **API Changes (Backward Incompatible)**
 
 -
diff --git a/src/hyperframe/frame.py b/src/hyperframe/frame.py
index a67487e..2807d68 100644
--- a/src/hyperframe/frame.py
+++ b/src/hyperframe/frame.py
@@ -132,7 +132,7 @@ def parse_frame_header(header: memoryview, strict: bool = False) -> tuple[Frame,
         length = (fields[0] << 8) + fields[1]
         typ_e = fields[2]
         flags = fields[3]
-        stream_id = fields[4] & 0x7FFFFFFF
+        stream_id = fields[4] & 0x7FFFFFFF # mask off the reserved bit, RFC 9113, Section 4.1
 
         try:
             frame = FRAMES[typ_e](stream_id)
@@ -172,7 +172,7 @@ def serialize(self) -> bytes:
             self.body_len & 0xFF,
             self.type,
             flags,
-            self.stream_id & 0x7FFFFFFF,  # Stream ID is 32 bits.
+            self.stream_id & 0x7FFFFFFF,  # mask off the reserved bit, RFC 9113, Section 4.1
         )
 
         return header + body
@@ -271,7 +271,7 @@ def parse_priority_data(self, data: memoryview) -> int:
             raise InvalidFrameError(msg) from err
 
         self.exclusive = bool(self.depends_on >> 31)
-        self.depends_on &= 0x7FFFFFFF
+        self.depends_on &= 0x7FFFFFFF  # mask off the exclusive bit, RFC 9113, Section 6.3
         return 5
 
 
@@ -620,7 +620,7 @@ def _body_repr(self) -> str:
 
     def serialize_body(self) -> bytes:
         data = _STRUCT_LL.pack(
-            self.last_stream_id & 0x7FFFFFFF,
+            self.last_stream_id & 0x7FFFFFFF, # mask off the reserved bit, RFC 9113, Section 6.8
             self.error_code,
         )
         data += self.additional_data
@@ -636,6 +636,8 @@ def parse_body(self, data: memoryview) -> None:
             msg = "Invalid GOAWAY body."
             raise InvalidFrameError(msg) from err
 
+        # mask off the reserved bit, RFC 9113, Section 6.8
+        self.last_stream_id = self.last_stream_id & 0x7FFFFFFF
         self.body_len = len(data)
 
         if len(data) > 8:
@@ -674,7 +676,9 @@ def _body_repr(self) -> str:
         return f"window_increment={self.window_increment}"
 
     def serialize_body(self) -> bytes:
-        return _STRUCT_L.pack(self.window_increment & 0x7FFFFFFF)
+        return _STRUCT_L.pack(
+            self.window_increment & 0x7FFFFFFF, # mask off the reserved bit, RFC 9113, Section 6.9
+        )
 
     def parse_body(self, data: memoryview) -> None:
         if len(data) > 4:
@@ -687,6 +691,9 @@ def parse_body(self, data: memoryview) -> None:
             msg = "Invalid WINDOW_UPDATE body"
             raise InvalidFrameError(msg) from err
 
+        # mask off the reserved bit, RFC 9113, Section 6.9
+        self.window_increment = self.window_increment & 0x7FFFFFFF
+
         if not 1 <= self.window_increment <= 2**31-1:
             msg = "WINDOW_UPDATE increment must be between 1 to 2^31-1"
             raise InvalidDataError(msg)
@@ -904,7 +911,7 @@ def serialize(self) -> bytes:
             self.body_len & 0xFF,
             self.type,
             flags,
-            self.stream_id & 0x7FFFFFFF,  # Stream ID is 32 bits.
+            self.stream_id & 0x7FFFFFFF,  # mask off the reserved bit, RFC 9113, Section 4.1
         )
 
         return header + self.body
diff --git a/tests/test_frames.py b/tests/test_frames.py
index 10f2300..87f66d5 100644
--- a/tests/test_frames.py
+++ b/tests/test_frames.py
@@ -647,6 +647,35 @@ def test_short_goaway_frame_errors(self):
         with pytest.raises(InvalidFrameError):
             decode_frame(s)
 
+    def test_goaway_frame_with_reserved_bit_set_parses_properly(self):
+        s = (
+            b'\x00\x00\x0D\x07\x00\x00\x00\x00\x00' +  # Frame header
+            b'\x80\x00\x00\x40' +                      # Last Stream ID with reserved bit set
+            b'\x00\x00\x00\x20' +                      # Error Code
+            b'hello'                                   # Additional data
+        )
+        f = decode_frame(s)
+
+        assert isinstance(f, GoAwayFrame)
+        assert f.flags == set()
+        assert f.additional_data == b'hello'
+        assert f.body_len == 13
+        assert f.last_stream_id == 64
+
+    def test_goaway_frame_with_reserved_bit_set_serializes_properly(self):
+        f = GoAwayFrame()
+        f.last_stream_id = 64
+        f.error_code = 32
+        f.additional_data = b'hello'
+
+        s = f.serialize()
+        assert s == (
+            b'\x00\x00\x0D\x07\x00\x00\x00\x00\x00' +  # Frame header
+            b'\x00\x00\x00\x40' +                      # Last Stream ID
+            b'\x00\x00\x00\x20' +                      # Error Code
+            b'hello'                                   # Additional data
+        )
+
 
 class TestWindowUpdateFrame:
     def test_repr(self):
@@ -694,6 +723,22 @@ def test_short_windowupdate_frame_errors(self):
         with pytest.raises(InvalidDataError):
             decode_frame(WindowUpdateFrame(2**31).serialize())
 
+    def test_window_update_frame_with_reserved_bit_set_parses_properly(self):
+        s = b'\x00\x00\x04\x08\x00\x00\x00\x00\x80\x00\x00\x02\x00'
+        f = decode_frame(s)
+
+        assert isinstance(f, WindowUpdateFrame)
+        assert f.flags == set()
+        assert f.window_increment == 512
+        assert f.body_len == 4
+
+    def test_window_update_frame_with_reserved_bit_set_serializes_properly(self):
+        f = WindowUpdateFrame(0)
+        f.window_increment = 512
+
+        s = f.serialize()
+        assert s == b'\x00\x00\x04\x08\x00\x00\x00\x00\x00\x00\x00\x02\x00'
+
 
 class TestHeadersFrame:
     def test_repr(self):