Market Exchange Rates (MER) balance the demand and supply for international currencies, while Purchasing Power Parity (PPP) exchange rates capture the differences between the cost of a given bundle of goods and services in different countries. When undertaking multi-country analysis of environmental issues (such as climate change) that includes different currencies, a decision has to be made as to whether to use PPP or MER in the analytical framework. The distinction between them is particularly germane in inter-temporal studies that postulate future scenarios. PPPs are generally favoured for their closer link to welfare, but MERS are necessarily the basis of international trade, so it is difficult to choose between them. Some authors have noted some empirical regularity between them and have sought to exploit this to avoid choosing between PPP and MER. In this paper, it is shown that such ad hoc adjustments are not necessary when structural changes are accounted for.