diff --git a/pyproject.toml b/pyproject.toml
index a4f224a..acefa33 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -36,6 +36,7 @@ keywords = [
 
 [tool.poetry.dependencies]
 python = ">=3.8"
+lxml = ">=4.8.0"
 
 [tool.poetry.group.dev.dependencies]
 pycodestyle = "*"
diff --git a/space_packet_parser/xtcedef.py b/space_packet_parser/xtcedef.py
index e7d1109..c403319 100644
--- a/space_packet_parser/xtcedef.py
+++ b/space_packet_parser/xtcedef.py
@@ -7,7 +7,8 @@
 import struct
 from typing import Tuple, Union, Optional, Any, List, TextIO, Dict
 import warnings
-from xml.etree import ElementTree
+# Installed
+import lxml.etree as ElementTree
 
 logger = logging.getLogger(__name__)
 
@@ -2322,7 +2323,6 @@ def __repr__(self):
 class XtcePacketDefinition:
     """Object representation of the XTCE definition of a CCSDS packet object"""
 
-    _default_namespace = {'xtce': 'http://www.omg.org/space/xtce'}
     _tag_to_type_template = {
         '{{{xtce}}}StringParameterType': StringParameterType,
         '{{{xtce}}}IntegerParameterType': IntegerParameterType,
@@ -2352,10 +2352,10 @@ def __init__(self, xtce_document: TextIO, ns: Optional[dict] = None):
         self._sequence_container_cache = {}  # Lookup for parsed sequence container objects
         self._parameter_cache = {}  # Lookup for parsed parameter objects
         self._parameter_type_cache = {}  # Lookup for parsed parameter type objects
-        self.ns = ns or self._default_namespace
+        self.tree = ElementTree.parse(xtce_document)
+        self.ns = ns or self.tree.getroot().nsmap
         self.type_tag_to_object = {k.format(**self.ns): v for k, v in
                                    self._tag_to_type_template.items()}
-        self.tree = ElementTree.parse(xtce_document)
 
         self._populate_sequence_container_cache()
 
diff --git a/tests/unit/test_xtcedef.py b/tests/unit/test_xtcedef.py
index 3fe9336..fd6db0c 100644
--- a/tests/unit/test_xtcedef.py
+++ b/tests/unit/test_xtcedef.py
@@ -1,8 +1,9 @@
 """Tests for space_packet_parser.xtcedef"""
 # Standard
-from io import StringIO
+import io
+# Installed
 import pytest
-from xml.etree import ElementTree
+import lxml.etree as ElementTree
 # Local
 from space_packet_parser import xtcedef, parser
 
@@ -38,9 +39,9 @@ def test_invalid_parameter_type_error(test_data_dir):
     </xtce:TelemetryMetaData>
 </xtce:SpaceSystem>
 """
-    x = StringIO(test_xtce_document)
+    x = io.TextIOWrapper(io.BytesIO(test_xtce_document.encode("utf-8")))
     with pytest.raises(xtcedef.InvalidParameterTypeError):
-        xtcedef.XtcePacketDefinition(x, ns=TEST_NAMESPACE)
+        xtcedef.XtcePacketDefinition(x)
 
 
 def test_unsupported_parameter_type_error(test_data_dir):
@@ -79,9 +80,9 @@ def test_unsupported_parameter_type_error(test_data_dir):
     </xtce:TelemetryMetaData>
 </xtce:SpaceSystem>
 """
-    x = StringIO(test_xtce_document)
+    x = io.TextIOWrapper(io.BytesIO(test_xtce_document.encode("utf-8")))
     with pytest.raises(NotImplementedError):
-        xtcedef.XtcePacketDefinition(x, ns=TEST_NAMESPACE)
+        xtcedef.XtcePacketDefinition(x)
 
 
 def test_attr_comparable():