Since Representational State Transfer (REST) architecture was proposed by Fielding in early 1990s for distributed hypermedia systems, it has become a popular architectural style of choice in various computing environments. However, REST was not originally designed to support enterprise requirements, in particular the accountability requirements that are crucial for the business services offered through the Software as a Service (SaaS) and Cloud Computing environments. In this paper, we propose an Accountable State Transfer (AST) architecture to bridge the accountability gap in REST. With AST, service participants can be held accountable for each representational state transfer during service consumption. A formal service contract model with a hybrid reasoning mechanism and a novel accountable state transfer protocol are designed as the mechanisms underpinning the AST architecture. Moreover, we implement a Credit Check service prototype based on AST, demonstrating the practicality of such architecture. Inheriting REST's scalability, AST architecture provides the much needed accountability capabilities for the virtual service delivery environment. © 2010 IEEE.