Inter-organizational knowledge networks and knowledge marketplaces have emerged in order to enable organizations to share or commercially exploit their knowledge outside narrow organizational borders. The materialization of these structures requires concrete and sound mechanisms for the efficient external provision of knowledge stored in knowledge repositories and for the conclusion of agreements among parties that will specify the terms and conditions of knowledge transactions. In our approach, we propose ?Knowledge Services? and k-UDDI, enhanced by reference ontologies we have developed, as a means to enable the publication, discovery, negotiation and invocation of web services that carry knowledge objects from knowledge repositories of various organizational contexts regardless of the environment within which they are delivered.