Among the problems facing designers of complex multi-participant Web services-based applications is dealing with the consequences of the lack of suitable isolation mechanisms. This deficiency means that concurrent applications can interfere with each other, resulting in race conditions and lost updates. This paper considers a proposed solution to this problem based on ?promises? and shows that this model can be implemented in practice. We consider implementation issues that need to be handled in promise-based systems and discuss a proof-of-concept prototype that supports promise-based isolation without requiring changes to existing applications and resources.