This article argues that middle power theories can be tested by looking at the practice of negotiating multilateral treaties on conventional weapons. Three recent treaties are noteworthy, regulating landmines, cluster munitions and the arms trade. They are noteworthy because they were negotiated through multilateral diplomacy by coalitions of countries often identified as middle powers, and resulted in humanitarian, effects-based regulations. Canada and Australia actively participated in all three and are regularly cited as exemplars of middle powers. On landmines and the arms trade, both behaved as middle power theories would anticipate, embracing multilateralism and seeking a principled outcome. On cluster munitions, neither country conformed to expectations and instead unsuccessfully sought an incremental approach in line with their US ally. These cases therefore offer promising terrain to test claims around middle powers, multilateralism, and principled approaches to world affairs. This article sheds light on these inconsistencies by identifying the contextual changes behind these differing postures through a 'contexts of diplomacy' framework. This article also highlights the strategies and tactics of Canada and Australia. This serves two purposes. First, it helps address some of the issues raised in middle power theories. Second, it helps identify modes of middle power diplomacy.