While fuel-cell technology is expected to contribute to mitigating CO2 emissions in Japan, no quantitative analyses of energy consumption and accompanying CO2 emissions have been performed. This study analyzes quantitatively the changes in them effected by the penetration of proton exchange membrane fuel cell (PEM-FC) cogeneration systems in Japan by taking account of the dynamic changes in the power-generation mix. The optimal generation mix model, developed by the Japanese Central Research Institute of the Electric Power Industry, is used for the calculations. The results show decreased and increased cases of CO2 emissions across the overall energy system, as compared with the reference cases. Key factors influencing these changes are the development of nuclear power, and technical progress in the efficiencies of PEM-FC systems and their operating patterns in the future. (c) 2004 Elsevier Ltd. All rights reserved.