Today, peer-to-peer services can comprise a large and growing number of services, e.g. search services or services dealing with heterogeneous schemas in the context of Digital Libraries. For a given task, the system has to determine suitable services and their processing order (execution plan). As peers can join or leave the network spontaneously, static execution plans are not sufficient. This paper proposes a logic-based approach for dynamically computing execution plans: Services are described in the DAML-S language. These descriptions are mapped onto Datalog. Finally, logical rules are applied on the service description facts for determining matching services and finding an optimum execution plan.