In this study, effects on a spark ignition engine of mixtures of hydrogen and methane have been experimentally considered. This article presents the results of a four-cylinder engine test with mixtures of hydrogen in methane of 0, 10, 20 and 30% by volume. Experiments have been made varying the equivalence ratio. Equivalence ratios have been selected from 0.6 to 1.2. Each fuel has been investigated at 2000 rpm and constant load conditions. The result shows that NO emissions increase, HC, CO and CO2 emission values decrease and brake thermal efficiency (BTE) values increase with increasing hydrogen percentage. © 2007 International Association for Hydrogen Energy.