diff --git a/introspection/org.freedesktop.ModemManager1.Modem.Modem3gpp.xml b/introspection/org.freedesktop.ModemManager1.Modem.Modem3gpp.xml
index b9e9b19d..205f7e21 100644
--- a/introspection/org.freedesktop.ModemManager1.Modem.Modem3gpp.xml
+++ b/introspection/org.freedesktop.ModemManager1.Modem.Modem3gpp.xml
@@ -200,7 +200,25 @@
     <!--
         Pco:
 
-        The raw PCOs received from the network.
+        The raw PCOs received from the network, given as array of PCO
+        elements (signature <literal>"a(ubay)"</literal>).
+
+        Each PCO is defined as a sequence of 3 fields:
+        <orderedlist>
+          <listitem>
+            The session ID associated with the PCO, given as an
+            unsigned integer value (signature <literal>"u"</literal>).
+          </listitem>
+          <listitem>
+            The flag that indicates whether the PCO data contains the
+            complete PCO structure received from the network, given as
+            a boolean value (signature <literal>"b"</literal>).
+          </listitem>
+          <listitem>
+            The raw  PCO data, given as an array of bytes (signature
+            <literal>"ay"</literal>).
+          </listitem>
+        </orderedlist>
     -->
     <property name="Pco" type="a(ubay)" access="read" />