The purpose of this study is to experimentally analyze the performance and the pollutant emissions of a four-stroke spark-ignition engine operating on natural gas-hydrogen blends of 0%, 10%, 20% and 30% at full load and 65% load for different excess 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 x stroke of 80.6 x 88 mm and a compression ratio of 10:1. Experiments were made at a constant engine speed of 2000 rpm. CO, CO2 and HC emission values and cylinder pressures were measured. The results showed that while the excess air ratio increases, CO and CO2 emission values decrease. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.