Cost negotiation for communication sessions

A method of negotiating the sharing of a cost associated with a communication service between plural participants. The method comprises sending a cost sharing request from a terminal 1 used by a first of the participants to the terminal 12,13 used by the or each other participant. At the or each terminal 12,13 receiving said request, the request is processed using predefined cost sharing rules and/or user input relating to the request received. The initiating terminal 1 is notified of the result. Messages exchanged between the participating terminals are intercepted within the network(s) 2,14 supporting the service, and the participants charged in accordance with the negotiated cost sharing formula.