The results of an experimental investigation involving the addition of hydrogen to a gasoline-fuelled SI engine are reported. Up to 66% by volume (3.7% by mass) of hydrogen as fuel was added as part of the air with little modification to the engine. Cylinder pressure traces were used to calculate the indicated mean effective pressure and mass fraction burned. Electrochemical analysers were used to measure the concentration of CO, NO and O(2) in the exhaust. The added hydrogen resulted in improved work output and a reduction in burn duration and cycle-to-cycle variation while operating under lean conditions (phi < 0.85). When operating closer to stoichiometric conditions (phi > 0.85) little difference in engine performance was seen. This dependence of hydrogen addition effect on the fuel/air equivalence ratio was confirmed by analysis of variance tests. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.