In this study, an experimental study on the performance and exhaust emissions of a spark-ignition engine fuelled with methane-hydrogen mixtures (100% CH4, 10% H2 + 90% CH4, 20% H2 + 80% CH4, and 30% H2 + 70% CH4) were performed at different engine speeds and different excessive air ratios. This present work was carried out on a Ford engine. This is a four-stroke cycle four-cylinder spark-ignition engine with a bore of 80.6 mm, a stroke of 88 mm and a compression ratio of 10:1. Experiments were performed at 1500, 2000, 2500 and 3000 rpm and at wide open throttle (WOT). CO, CO2 and HC emission values and cylinder pressure were measured. The results showed that while the speed and excessive air ratio increase, CO emission values decrease. The reduction of HC and CO emissions could be obtained by adding hydrogen into the natural gas when operating on the lean mixture condition. Increasing the excessive air ratio also decreases the maximum peak cylinder pressure. © 2008 International Association for Hydrogen Energy.