We investigated the generating efficiency and pollutant emissions of a four-stroke spark-ignition gas engine generator operating on biogas-hydrogen blends of varying excess air ratios and hydrogen concentrations. Experiments were carried out at a constant engine speed of 1200 rpm and a constant electric power output of 10 kW The experimental results showed that the peak values of genera ring efficiency, maximum cylinder pressure, and NO(x) emissions were elevated at an excess air ratio of around 1.2 as the hydrogen concentration was increased. CO(2) emissions decreased as the excess air ratio and hydrogen concentration increased, due to lean-burn conditions and hydrogen combustion. An efficiency per NO(x) emissions ratio (EPN) was defined to consider the relationship between the generating efficiency and NO(x) emissions. A maximum EPN value of 0 7502 was obtained with a hydrogen concentration of 15%, for an excess air ratio of 2 0. At this EPN value, the NO(x) and CO(2) emissions were 39 ppm and 1678 32 g/kWh, respectively, and the generating efficiency was 29.26% These results demonstrated that the addition of hydrogen to biogas enabled the effective generation of electricity using a gas engine generator through lean-bum combustion. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.