In the standard economic model of cap and trade policies, the regulator is assumed to place zero value on pollution reductions below the cap. This paper considers an alternative case, where the policy makers can manipulate the rules of the program to achieve improved environmental performance. This is achieved by manipulating the trading ratio, the units of pollution credits that are obtained for each unit of pollution reduction. Using a parsimonious model of a transferable discharge permits program, we identify the environmentally optimal trading ratio that maximizes the environmental gains of trading. The model suggests an alternative explanation why non-unitary trading ratios are common and is a counterpoint to the cost-minimizing model that predominates in economics. We conclude by recommending that a middle-ground should be sought, where both environmental gains and cost efficiencies are given weight.