Hi, I am sure I am just missing something obvious, but here goes! I have a document like <http://kjc-fs2.kjc.uni-heidelberg.de:8600/exist/apps/tamboti/modules/search/source.xql?id=uuid-b0de7aeb-c208-4290-8466-1bb0d9af5172&clean=yes> When I simply "Validate" this, that is click the red checkmark, I get SystemID: http://cluster-schemas.uni-hd.de/modsCluster.xsd Main validation file: /Users/jens/Desktop/Untitled1.xml Schema: http://cluster-schemas.uni-hd.de/modsCluster.xsd Engine name: Saxon-EE 9.3.0.5 Severity: fatal Description: The attributeGroup {simpleLink} is referenced, but has not been declared Start location: 1051:0 … The MODS schema in question references the XLink schema which declares the xlink:simpleLink attributeGroup, but the link to the XLink schema is obviously not de-referenced. However, when I choose "Validate with …" by clicking on the arrow to the right of the checkmark and feed the schemaLocation contained in the MODS document (namely, <http://cluster-schemas.uni-hd.de/modsCluster.xsd>), the document validates. In this case then, the XLink schema must be referenced. What explains this difference? When I open the schema at <http://cluster-schemas.uni-hd.de/modsCluster.xsd>, the XLink schema is also not de-referenced. Best, Jens oXygen 13.0 on Max OS X 10.6.8.