Marketable emission permit mechanisms are increasingly proposed as efficient means of managing environmental pollution problems such as greenhouse gas emissions. Existing examples of emissions trading in the literature have so far been limited to domestic efforts put in place through the action of a national legislature, which has no parallel in international politics. This paper examines two efforts to establish multi-lateral emissions trading for nitrogen oxides among various states with the US. One, the Ozone Transport Commission's NOx Budget program is a success. The other, the Ozone Transport Assessment Group and the federal government's subsequent NOx SIP Call has not resulted in a multi-lateral emissions control program, let alone an efficient, market-based one. Due to the relative similarities of the states (compared to highly heterogeneous nations of the world) these are "best case" examples, and explaining the vast differences in outcomes will help explain the potential and the challenges in developing an international emission trading program to control greenhouse gas emissions.