In this paper, the authors describe UMIS, a service architecture that enables user adaptive applications to exchange User Model data on the Web. UMIS provides a set of facilities that allow applications to interoperate with minimum changes in their internal logics and knowledge representation. The goal is to support the process of interoperability in three ways: providing an efficient centralized discovery service; offering a service for simple interaction for the exchange of UM value in a p2p way and offering a negotiation mechanism to be used in case of communication hurdles (i.e. semantic ambiguities and missing response). They developed a proof-of-concept prototype of UMIS and they tested it with an existing user-adaptive application.