diff --git a/resources/schema/inventory.xsd b/resources/schema/inventory.xsd
index 125b3a6..59417f3 100644
--- a/resources/schema/inventory.xsd
+++ b/resources/schema/inventory.xsd
@@ -69,7 +69,13 @@
   <xsd:complexType name="lexemeType">
     <xsd:simpleContent>
       <xsd:extension base="xsd:string">
-        <xsd:attribute name="type" type="xsd:string"/> <!-- hmm: should probably restrict possible values? currently seems to be "compound" or empty -->
+        <xsd:attribute name="type"> <!-- hmm, what types are expected; currently either missing or "argument", but does that really make sense? -->
+          <xsd:simpleType>
+            <xsd:restriction base="xsd:string">
+              <xsd:enumeration value="argument"/>
+            </xsd:restriction>
+          </xsd:simpleType>
+        </xsd:attribute>
         <xsd:attribute name="lexical_unit_lexeme_id" type="xsd:int"/> <!-- ID within its lexical unit -->
         <xsd:attribute name="sloleks" type="xsd:string"/> <!-- Sloleks ID -->
         <xsd:attribute name="kol" type="xsd:string"/> <!-- hmm, legacy attribute for a collocate's lemma; why not just use @lemma? -->  
@@ -88,7 +94,14 @@
       </xsd:choice>
     </xsd:sequence>
     <xsd:attribute name="id" type="xsd:int" use="required"/> 
-    <xsd:attribute name="type" type="xsd:string" use="required"/> <!-- should probably restrict possible values in line with superbaza lexical_unit_type -->
+    <xsd:attribute name="type" use="required"> <!-- types of headwords allowed; hmm, probably should standardise allowed settings -->
+      <xsd:simpleType>
+        <xsd:restriction base="xsd:string">
+          <xsd:enumeration value="single"/>
+          <xsd:enumeration value="MWE"/>
+        </xsd:restriction>
+      </xsd:simpleType>
+    </xsd:attribute>
     <xsd:attribute name="structure_id" type="xsd:int"/> <!-- syntactic structure ID -->
     <xsd:attribute name="origin" type="xsd:string"/> <!-- legacy attribute to track where the lexical unit came from -->
   </xsd:complexType>
@@ -123,7 +136,7 @@
   <xsd:complexType name="measureType">
     <xsd:simpleContent>
       <xsd:extension base="xsd:decimal">
-        <xsd:attribute name="type" type="xsd:string" use="required"/> <!-- type of measure (e.g., frequency, logDice) -->
+        <xsd:attribute name="type" type="xsd:string" use="required"/> <!-- type of measure (e.g., frequency, logDice); hmm, probably should be enumerated -->
         <xsd:attribute name="source" type="xsd:string"/> <!-- context of the measure, probably corpus name and version -->
       </xsd:extension>
     </xsd:simpleContent>
@@ -250,7 +263,7 @@
     <xsd:sequence>
       <xsd:element name="comp" type="compType" maxOccurs="unbounded"/> <!-- the tree contains one or more components -->
     </xsd:sequence>
-    <xsd:attribute name="role" type="xsd:string"/> <!-- should probably be a valid value from semanticRoleType -->
+    <xsd:attribute name="role" type="semanticRoleType"/>
   </xsd:complexType>
 
   <!-- Example sentence from a corpus (as opposed to a multi-word example) -->
@@ -393,7 +406,14 @@
     <xsd:sequence>
       <xsd:element name="syntacticStructure" type="syntacticStructureType" minOccurs="0" maxOccurs="unbounded"/>
     </xsd:sequence>
-    <xsd:attribute name="system" type="xsd:string"/> <!-- probably JOS or UD -->
+    <xsd:attribute name="system"> <!-- the system within which the structure is defined  -->
+      <xsd:simpleType>
+        <xsd:restriction base="xsd:string">
+          <xsd:enumeration value="JOS"/>
+          <xsd:enumeration value="UD"/>
+        </xsd:restriction>
+      </xsd:simpleType>
+    </xsd:attribute>
   </xsd:complexType>
 
   <!-- Container associating a semantic role with a list of syntactic structures -->