As part of the MKBEEM project, we present an ontology based mediation framework for electronic commerce applications. The framework is based on a mediator /wrapper approach that supports an integrated view over multiple heterogeneous sources. The MKBEEM mediation system allows to fill the gap between customers queries (possibly expressed in a natural language) and diverse specific providers offers. In contrast with many existing mediator based systems, our approach rests on a three-layer knowledge representation architecture which includes an electronic services ontology besides the usual domain ontology and sources descriptions layers. At the reasoning level, we propose a new mechanism, namely dynamic discovery of e-services, that acts in collaboration with the Picsel mediator system to effectively achieve the MKBEEM mediation tasks.