Dynamic coalitions formed by business partners who may be competitors but yet still need to co-operate with each other require independent, secure and reliable storage for exchanging and sharing data among its members. This paper presents a virtual storage services architecture that addresses these requirements by abstracting physical storage space in order to provide a pool of logical storage space using virtualisation technique. The abstraction of a variety of physical storage technologies and their capabilities is achieved by designing an XML-based simple distributed storage interface (SDSI). This paper also presents a case study of a secure distributed storage service to demonstrate how our architecture offers secure data storing and sharing for dynamic coalitions. We present a summary of our implementation, a Web Services-based prototype storage system using SDSI, and evaluated its performance against a number of alternative distributed storage technologies and data transport protocols.