An increasing level of cooperation between public administrations nowadays on national, regional and local level requires methods to develop interoperable E- Government systems and leads to the necessity of an efficient modeling of collaborative business processes and their implementation. In this paper a framework to systematically develop interoperable systems in the public sector is described. The methodology is motivated and explained on the basis of an E-Government reference model for the German Plan Approval Procedure. In this context it is described how cross-organizational business processes can be modeled and transformed to technical process models in the form of Web Service protocols. After a motivation for the need of interoperability frameworks and a description of related work, section 3 introduces the methodical interoperability framework. Further detailing the contents of this framework, section 4 presents a method for the creation of collaborative business processes on a conceptual level. Section 5 changes to the execution level and describes how to implement the conceptual models created before with Web Service protocols. Section 6 concludes the paper and gives an outlook on future research.