Opening public sector information has recently become a trend in many countries around the world. Online government data catalogues with national, regional or local scope act as one-stop data portals providing descriptions of available government datasets. These catalogues though remain isolated. Potential benefits from federating geographically overlapping or thematically complementary catalogues are not realized. We propose an RDF Schema vocabulary as an interchange format among data catalogues and as a way of bringing them into the Web of Linked Data, where they can enjoy interoperability among themselves and with other deployed datasets. The vocabulary's design was informed by a survey of seven data catalogues from five different countries, and has been verified by unifying four data catalogues to allow cross-catalogue queries and browsing.