The Web Services Transaction protocol family includes the WS-AtomicTransaction and the WSBusinessActivity specifications in order to carry out distributed transactions in a Web Services (WS) environment. The WS-AtomicTransaction specification defines all necessary interfaces to carry out transactional work. In contrary, the WS-BusinessActivity specification for long-running transactions intentionally left the interface between initiator and coordinator undefined. This allows vendors to integrate WS-BusinessActivity coordinators into their business process engines. However, it requires proprietary protocols between initiator and coordinator. We propose an extension protocol to the WS-BusinessActivity specification that explicitly defines this interface between initiator and coordinator. This extension allows coordinators and initiators from different vendors to interoperate transparently. Accordingly, participants no longer need to trust an initiator-selected and likely initiator-run coordination service, but may use commonly trusted, third-party coordination services.