Most of the information systems (IS) interoperability research and practice in the e-government area has been focused on the operational level, aiming mainly at enabling the delivery of integrated electronic services involving several government agencies to citizens and enterprises based on the ‘one-stop shop’ model, and the support of co-operation among government agencies from the same or even different countries at the operational level. This paper is dealing with knowledge-level interoperability, which aims at the support of higher knowledge-intensive tasks of government, such as the formulation of legislation. In particular, it presents an ontology-based methodology for achieving knowledge-level interoperability among IS of Parliaments. It is based on the common use by Parliaments of the ontology of the ‘Issue-Based Information Systems’ (IBIS) framework for codifying the public policy related knowledge produced in the various stages of legislation formulation. Also, an application of the proposed methodology is presented for the case of the Law concerning ‘Contracts of Voluntary Cohabitation’ that has been recently passed by the Greek Parliament; its evaluation resulted in a proposal for a refinement of the above ontology that can be used for achieving a better codification of the knowledge that the main content (articles) of Laws contains.