Abstract. Combinatorial Auctions are an attractive application of intelligent agents; their applications are countless and are shown to provide good revenues. On the other hand, one of the issues they raise is the computational complexity of the solving process (the Winner Determination Problem, WDP), that delayed their practical use. Recently, efficient solvers have been applied to the WDP, so the framework starts to be viable. A second issue, common to many agent systems, is trust: in order for an agent system to be used, the users must trust both their representative and the other agents inhabiting the society. Malicious agents must be found, and their violations discovered. The SOCS project addresses such issues, and provided a language, the social integrity constraints, for defining the allowed interaction moves, together with a proof-procedure able to detect violations. In this paper we show how to write a protocol for the combinatorial auctions by using social integrity constra...