This paper studies argumentation-based dialogues between agents. It takes a previously defined system by which agents can trade arguments and examines the outcomes of the dialogues this system permits. In addition to providing a first characterisation of such outcomes, the paper also investigates the extent to which outcomes are dependent on tactical play by the agents, and arguing that this violates principles of mechanism design, identifies how to prevent tactics having an effect. Categories and Subject Descriptors I.2.11 [Artificial Intelligence]: Distributed Artificial Intelligence—Coherence and co-ordination; multiagent systems. General Terms Languages, theory. Keywords Agent communication, dialogue games, argumentation.