Catalog support page doesn't describe how a list of directories is formatted in XML_CATALOG_FILES
http://xmlsoft.org/catalog.html speaks of "setting the XML_CATALOG_FILES environment variable to a list of catalogs" but does not explain that the list elements are space-separated. The sole example contains only one catalog. I assumed that this was colon-separated (as PATH is) and of course resolution failed. I eventually discovered the proper formatting by experiment.