Renewable energy and greenhouse gas emissions markets are currently in a state of confusion regarding the treatment of Renewable Energy Certificate (RECs). How should emission-trading schemes treat RFCs? How can emission mitigation policies provide real incentives for renewable generation? The objective of REC markets should be to promote additional renewable energy investments. The author asserts that defining RECs in terms of attributes, especially off-site attributes, does not further this goal. Ambiguous language such as "environmental attribute" or "environmental benefit" creates confusion in the marketplace while failing to address the relevant coordination issues with Renewable Portfolio Standard compliance markets, voluntary emission offset markets, or emission cap-and-trade markets. Specifically, defining RECs in terms of off-site attributes creates a number of problems, including that once an emissions cap-and-trade scheme is in place, such definitions of a REC can become indefensible. The author proposes to redefine RECs in terms of on-site attributes, which resolves the aforementioned problems and allows compliance and voluntary renewable energy and emission markets to function without conflicts. Ideally, environmental commodities should be homogeneous, first best measures of the relevant environmental good, as well as easily measured and verified. The author proposes tradable environmental commodities that achieve these characteristics. (C) 2008 Elsevier Ltd. All rights reserved.