This chapter offers an explanation based on a number of interconnected meanings of value. The UN Kyoto Protocol established the Clean Development Mechanism and Joint Implementation as ‘flexibility mechanisms’ that enabled carbon offsetting as part of the international policy response to climate change. The chapter argues that a primary driver for the decline in offset markets is the growing perception that offsets have limited use value as a result of their failure to provide the intended quantified carbon emissions reductions. An offset mechanism must discriminate between those projects which simply reduce emissions, and those which reduce emissions because of offsetting. To produce an environmental commodity like a carbon offset, one must measure some use value and attribute it to the commodity. The case of restricting offsets in the European Union Emissions Trading System shows that policy-makers came to have negative opinions about the worth of offsets in terms of their environmental, developmental, political and economic value.