The emerging standards for the specification of Web Services support the publication of the static interfaces of the operations they may execute. However, little attention is paid to the management of long-lasting interactions between the service providers and their consumers. Although this is not an issue in the case of "one-shot" services, it challenges the provision of services requiring the exchange of multiple messages between the business partners. In this paper, we present a conversation model supporting the management of longlasting interactions where several messages have to be exchanged before the service is completed. Our model aims at facilitating the consumers during the service invocation because in this way the establishment of short-term business relations can be simplified. To this extent, we provide a computational framework that can be exploited to manage a conversation between the consumer and the service provider. Our framework is inspired from the resea...