The limitations in the world sources of energy are being mitigated by the exploitation of renewable forms and by increases in the efficiency of energy utilization. Exergy analysis is a useful method for the design, evaluation, and improvement of energy systems, that uses conservation of mass and conservation of energy principles, together with the second law of thermodynamics. This study covers first and second law analyses of a cogeneration system run with the biogas produced in a landfill. Such plant produces useful electrical and thermal energies, while protecting the environment from greenhouse emissions. The objectives were to identify locations where major irreversibilities occur, to evaluate their magnitudes, and to assess the energy and exergy efficiencies of the global system and of its constituent units. The results show that the overall-plant first law efficiency is 37.9% and the exergy efficiency is 362%, which is far from the thermodynamic ideal limit. The internal combustion engine and one of the radiators are the most inefficient units, as judged by the parameters degree of thermodynamic perfection and exergy destruction quotient. The main potential for improvement in the plant is the harnessing of the energy in the exhaust gases. (C) 2013 Elsevier Ltd. All rights reserved.