Storing hydrogen is one of the major problems concerning its utilization on board vehicles. Today hydrogen can be compressed and stored at 200 or 350 bar (it is foreseen that in a near future storage pressure will reach 700 bar, according to new expected regulations and using tanks in composite materials) or cryogenically liquefied.An alternative solution is storing hydrogen in the form of ammonia that is liquid at roughly 9 bar at environmental temperature and therefore involves relatively small masses and volumes and requires light and low-cost tanks. Moreover, ammonia contains almost 18% hydrogen by mass and, by volume, liquid ammonia contains 1.7 times as much hydrogen as liquid hydrogen.It is well known that ammonia can be burned directly in I.C. engines, however a combustion promoter is necessary to support combustion especially in the case of high-speed S.I. engines. Among the potential promoters, hydrogen is worthy of note, since it is carbon free and counteracts ammonia combustion characteristics. As a matter of fact, hydrogen has high combustion velocity and wide flammability range, whereas ammonia combustion is characterized by low flame speed, low flame temperature, narrow flammability range, high ignition energy and high self-ignition temperature.The experimental activity shown in this paper is correlated with a project that is focused on a range-extended electric vehicle involving an ammonia-plus-hydrogen I.C. engine and where hydrogen is obtained from ammonia by means of on-board catalytic reforming. Accordingly, the test engine is a 505 cm₃ Lombardini twin-cylinder S.I. engine that is well suited to power the onboard electric generator and the activity is aimed at determining proper air-ammonia-hydrogen mixture compositions at actual operating speeds and loads of the engine connected to the electric generator.Hydrogen and ammonia are separately injected in the gaseous phase. The only mechanical modification of the engine involves the intake manifold, where electro-injectors for hydrogen and for ammonia (conventional ones for CNG application with appropriate modification to inner parts) are added to the original ones for gasoline.The experimental results confirm that it is necessary to add hydrogen to air-ammonia mixture to improve ignition and to increase combustion velocity, with ratios that depend mainly on load and less on engine speed. Brake power is less than with gasoline, due to mixture poor volumetric heating value and to ammonia low flame speed that penalizes engine brake thermal efficiency. The amount of hydrogen needed by the engine is compatible with the flow rate provided by the reformer, except at cold start. The maximum NOx emission is 11.5 g/kWh at half load and 4500 rpm, without catalytic reduction.