To remain competitive in the aggressive global market, the tourism industry and service providers face the need to collaborate and share their expertise and resources, as well as their costs and risks. The paradigm of the virtual enterprise addresses the necessary horizontal infrastructure needed to support such a collaboration framework. Designing a Web-based Tourism Information System (WTIS) as a virtual enterprise requires the proper infrastructure that supports the enterprises' inter-operation, task sharing, and federated information management. This paper addresses the main requirements and research challenges, and describes a reference architecture for the WTIS, focusing on a novel horizontal infrastructure to support the Virtual Tourism Enterprises and provide the necessary basis for vertical value-added-services developed on top of the infrastructure.