The combustion characteristics of hydrogen compared to gasoline offer the potential of an increased engine efficiency, especially at part load. Here, results are presented of the brake thermal efficiency of a bi-fuel hydrogen/gasoline engine, at several engine speeds and loads. Results on hydrogen are compared to results on gasoline. Hydrogen offers the possibility of a more flexible load control strategy. Where possible, results are compared between the wide open throttle, lean burn strategy and the throttled stoichiometric strategy. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.