Many concepts and systems for the technical support of communication, cooperation, and coordination of workgroups have been developed in the research field of computer-supported cooperative work over the last two decades. Yet, spontaneous coordination with remote parties can still be a challenge. Instant messaging tools improve the situation by providing users with presence and availability information about other users, and by supporting spontaneous text chat among co-present users. In this paper we present PRIMI (Platform for Research on Instant Messaging Infrastructures) an open and flexible platform that provides software developers with powerful, yet easy to use support for the implementation of novel concepts for instant messaging infrastructures. It offers the core functionalities for the development of instant messaging infrastructures; and it uses a sophisticated plug-in mechanism for adding user interfaces and communication protocols, as well as logging mechanisms.