In this paper we address the problem of semantic heterogeneity among agents during the exchange of messages in negotiation process execution. Traditional negotiation systems impose several requirements on the type and format of negotiation messages, as a consequence negotiation agents have to be reprogrammed according to the protocol and message specifications. In contrast, we present a solution based on the development of a negotiation ontology, which incorporates a shared vocabulary of the terms and messages for negotiating; and a semantic disambiguation module, that is invoked when a lack of understanding among agents occurs. We designed and implemented a service oriented architecture for executing negotiations and conducted experiments incorporating different negotiation messages. The tests showed that the proposed ontology improves the execution of negotiation messages.