This volume constitutes the refereed proceedings of the ninth International Conference on Co-operative Information Systems, CoopIS 2001, held in Trento, Italy in September 2001. The 29 revised full papers presented together with three invited contributions were carefully reviewed and selected from 79 submissions. The papers are organized in sections on: agent and systems; information integration; middleware, platforms, and architectures; models; multi and federated database systems; Web information systems; workflow management systems; and recommendation and information seeking systems.