This paper describes a mediation service (e.g. Trader service), offering to a client (e.g. an object) the possibility to ask the negotiator a service, processing the function that this service will have to accomplish. Our approach aims for a solution based on the functionality of ODP Trader but customised to CORBA. The emphasis of the work is on the solution of problems presented by an open distributed environment. Specifically, our work addresses the handling of: (i) heterogeneity of data; (ii) extension of the existing proposition concerning the IDL with semantic specification allowing the definition of properties on the types of objects and on the disponible services; (iii) determination if two specifications (type descriptions) are related by taking in account the behavior of the types. The paper also describes some details of a prototype for such a mediation service.