Abstract. Interoperability of heterogeneous networked systems has yet to reach the maturity required by ubiquitous computing due to the technology-dependent nature of solutions. The Connect Integrated Project attempts to develop a novel network infrastructure to allow heterogeneous networked systems to freely communicate with one another by synthesising the required connectors on-the-fly. A key objective of Connect is to build a comprehensive theory of composable connectors, by devising an algebra for rigorously characterising complex interaction protocols in order to support automated reasoning. With this aim in mind, we formalise a high-level algebra for reasoning about protocol mismatches. Basic mismatches can be solved by suitably defined primitives, while complex mismatches can be settled by composition operators that build connectors out of simpler ones. The semantics of the algebra is given in terms of Interface Automata, and an example in the domain of instant messaging is used to illustrate how the algebra can characterise the interaction behaviour of a connector for mediating protocols.