Interest-based negotiation (IBN) is a form of negotiation in which agents exchange information about their underlying goals, with a view to improving the likelihood and quality of a deal. While this intuition has been stated informally in much previous literature, there is no formal analysis of the types of deals that can be reached through IBN and how they differ from those reachable using (classical) alternating offer bargaining. This paper bridges this gap by providing a formal framework for analysing the outcomes of IBN dialogues, and begins by analysing a specific IBN protocol.