In this experimental investigation the affect of hydrogen addition to a landfill gas-fueled naturally-aspirated spark-ignition engine was explored. Hydrogen concentrations of 0%, 30%, 40%, and 50% by volume were added to simulated landfill gas (60% CH(4) and 40% CO(2))Efficiency, coefficient of variance of indicated mean effective pressure, and CO emissions were measured from near stoichiometric mixtures up to the lean operating limit. Engine-out NOx emissions were compared to predicted future best available control technology targets for NOx emissions in landfill gas-to-energy projects. From this study, it was determined that with 40% hydrogen by volume untreated exhaust NOx emissions can meet the 0.22 g/kWh NOx target while retaining 95% of baseline power and low CO emissions. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.