In the context of economic instruments for more energy efficiency and climate protection, tradable certificates have been investigated for renewable energy and for a number of emissions. In contrast, tradable energy efficiency - or "white" - certificates have only lately been considered as a market-based tool to foster energy efficiency as compared to standards and labelling, for example. Theoretically, there is little doubt about the advantages. In practice, however, some fundamental problems arise. Critical issues are the design of an efficient artificial market for "white" certificates, its compatibility with the European emissions trading system, the identification of a suitable target group for an energy efficiency obligation and the measurement of energy savings as compared to a reference use of energy. We use the theoretical framework of Transaction Cost Economics to elaborate these issues. We conclude that transaction costs and investment specificity will restrict markets for "white" certificates in practise. Long-term contracts rather than spot trade will be the prevailing form of governance for energy efficiency investments.