A compact onboard hydrogen generator has been developed for use with a hydrogen-enriched gasoline internal combustion engine. The unit uses gasoline and air in a partial oxidation reactor to produce a gaseous product containing hydrogen, carbon monoxide, minor amounts of methane, carbon dioxide and water, and nitrogen. A study of the theoretical equilibrium product composition has indicated an optimum operating point at an air/fuel ratio of 5.15, where a hydrogen/fuel mass ratio of 0.136 can be obtained under soot-free conditions. This is based on a gasoline with an atomic hydrogen to carbon ratio of 1.92.Both thermal and catalytic reactors have been tested. The thermal unit requires a reaction temperature of 2400°F to obtain 80% of the theoretical hydrogen yield. Soot formation tends to be a problem. The catalytic reactor yields close to theoretical yields at an operating temperature of 1800°F without any soot formation. A commercial nickel catalyst is used. A 100 h test with the catalytic unit showed no signs of performance degradation, using fully leaded Indolene 30.The calculated effect of hydrogen generator operating conditions on the fuel efficiency of a generator/engine combination is presented.