Owing to its widespread diffusion and ease of use, the Web is an attractive platform to support distributed cooperative work. The paper presents an object-oriented proxy-based framework to support synchronous cooperation on the Web. The framework implements the general-purpose mechanisms of a proxy server. Specific application-dependent functionalities have to be implemented in a module to be installed in the proxy-framework. This leads to the implementation of an extensible proxy-framework, which facilitates the development of specific cooperative services. The paper presents and evaluates several applications enabling different cooperative Web activities, to confirm the effectiveness of our proposal in terms of reusability, usability and performance. Copyright