diff --git a/base_classes/NXidentifier.nxdl.xml b/base_classes/NXidentifier.nxdl.xml
new file mode 100644
index 0000000000..ce05800f9e
--- /dev/null
+++ b/base_classes/NXidentifier.nxdl.xml
@@ -0,0 +1,49 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<?xml-stylesheet type="text/xsl" href="nxdlformat.xsl"?>
+<!--
+# NeXus - Neutron and X-ray Common Data Format
+#
+# Copyright (C) 2014-2024 NeXus International Advisory Committee (NIAC)
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License as published by the Free Software Foundation; either
+# version 3 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+#
+# For further information, see http://www.nexusformat.org
+-->
+<definition xmlns="http://definition.nexusformat.org/nxdl/3.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" category="base" type="group" name="NXidentifier" extends="NXobject" xsi:schemaLocation="http://definition.nexusformat.org/nxdl/3.1 ../nxdl.xsd">
+    <doc>
+         An identifier for a (persistent) resource, e.g., a DOI or orcid.
+    </doc>
+    <field name="service" type="NX_CHAR">
+        <doc>
+             The service by which the resource can be resolved.
+             
+             Examples: doi, urn, hdl, purl, orcid, iso, url
+        </doc>
+    </field>
+    <field name="identifier" type="NX_CHAR">
+        <doc>
+             The unique code, IRI or hash to resolve this reference.
+             Typically, this is stated by the service which is considered a complete
+             identifier, e.g., for a DOI it's something of the form `10.1107/S1600576714027575`
+             or `https://doi.org/10.1107/S1600576714027575`, which are both resolvable.
+        </doc>
+    </field>
+    <field name="is_persistent" type="NX_BOOLEAN">
+        <doc>
+             True if the identifier is persistent (i.e., unique and available indefinitely),
+             False otherwise.
+        </doc>
+    </field>
+</definition>
diff --git a/base_classes/NXsubentry.nxdl.xml b/base_classes/NXsubentry.nxdl.xml
index 2a95f45019..726080c202 100644
--- a/base_classes/NXsubentry.nxdl.xml
+++ b/base_classes/NXsubentry.nxdl.xml
@@ -83,7 +83,7 @@
 	<field name="title">
 		<doc>Extended title for entry</doc>
 	</field>
-	<field name="experiment_identifier">
+	<field name="experiment_identifier" type="NXidentifier">
 		<doc>
 			Unique identifier for the experiment, defined by
 			the facility, possibly linked to the proposals
@@ -95,13 +95,13 @@
 	<group type="NXnote"  name="experiment_documentation">
 		<doc>Description of the full experiment (document in pdf, latex, ...)</doc>
 	</group>
-	<field name="collection_identifier">
+	<field name="collection_identifier" type="NXidentifier">
 		<doc>User or Data Acquisition defined group of NeXus files or :ref:`NXentry`</doc>
 	</field>
 	<field name="collection_description">
 		<doc>Brief summary of the collection, including grouping criteria.</doc>
 	</field>
-	<field name="entry_identifier">
+	<field name="entry_identifier" type="NXidentifier">
 		<doc>unique identifier for the measurement, defined by the facility.</doc>
 	</field>
 	<field name="definition">
@@ -185,5 +185,4 @@
 	<group type="NXdata" />
 	<group type="NXparameters" />
 	<group type="NXprocess" />
-</definition>
-
+</definition>
\ No newline at end of file