The use of hydrogen as fuel represents a possible solution to reduce greenhouse gas emission from vehicles. Although proper engine running with hydrogen has been widely demonstrated, hydrogen storage onboard of the vehicle is a major problem. A promising solution is storing hydrogen in the form of ammonia that is liquid at roughly 9 bar at environmental temperature and therefore involves relatively small volumes and requires light and low-cost tanks. Ammonia can be burned directly in IC engines, however a combustion promoter is necessary to support combustion. As a matter of fact, the best (and carbon-free) promoter is hydrogen, which has very high combustion velocity and wide flammability range, whereas ammonia combustion is characterised by low flame speed and temperature, narrow flammability range and high ignition energy. The experimental activity shown in this paper is focused on analysing the behaviour of a 4-stroke twin-cylinder SI engine of 505 cm(3) fuelled ammonia-plus-hydrogen. This engine was expressly chosen and modified to be placed in a range extended vehicle where hydrogen is obtained from ammonia by means of on-board catalytic reforming. 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. Owing to intake air dilution and to lower flame speed of the ammonia-hydrogen-air mixture, a general decrement in engine performances has been detected in respect to the original gasoline version. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.