In this paper we present the concept of activity-centric collaboration using service-oriented architectures (ACCUSO), which addresses the requirements arising from ad-hoc collaboration in mobile teams. In ACCUSO, activities are used to map human actions to Web services exploiting the potential benefits of SOA, such as service discovery and binding at run time. The possibility to compose activities hierarchically from sub-activities and to redesign running activities provides the process-flexibility required in ad-hoc collaboration. We expand the notion of service orientation by introducing human-provided services (HpS) which provide functionality not realizable through software services. HpS are "implemented" by human actors (possibly being mobile), which remains transparent to the system, thereby allowing for the provisioning of HpS based on conventional WS-infrastructure. The feasibility and applicability of ACCUSO is demonstrated through a proof-of-concept implementation.