In its current state, service-oriented grid computing focuses on the unification of resources through virtualization, to enable on demand distributed computing within a preconfigured environment. Organizations or inter-organizational communities willing to share their computational resources typically create a centrally planned grid, where dedicated grid administrators manage the nodes and the offered grid services. In this paper, we present the idea of a spontaneously formed, service-oriented ad hoc grid to harness the unused resources of idle networked workstations and high-performance computing nodes on demand. We discuss the requirements of such an ad hoc grid, show how the service-oriented computing paradigm can be used to realize it and present a proof-of-concept implementation based on the Globus Toolkit 3.0. The features of this system are peer-to-peer based node discovery, automatic node property assessment, hot deployment of services into a running system and added inter-service security.