The topic of agent-based service composition has been experiencing much attention recently. Researchers are applying agent technology with the aim to improve adaptiveness and flexibility of prevailing static Web service composition solutions. One major characteristic of multi-agent systems in particular is their ability of emergent behavior that allows gaining complex system behavior from small distributed sets of simple rules. This paper describes a multi-agent-based coalition formation approach for service composition that achieves emergent behavior based on a light-weight interaction protocol and decentralized decision making. The paper also presents evaluation results of first experiments to underline the validity of the approach.