Cost negotiation for communication sessions

A method of negotiating the sharing of a cost associated with a communication service between a plurality of participants. The method comprises sending a cost sharing request from a terminal used by a first of the participants to the terminal used by the or each other participant At the or each terminal receiving said request, the request is processed using predefined cost sharing rules and/or user input relating to the request received. The initiating terminal is notified of the result. Messages exchanged between the participating terminals are intercepted within the network(s) supporting the service, and the participants charged in accordance with the negotiated cost sharing formula.