8 The mitigation of climate change implies the increasing use of variable renewable energy sources. Energy 9 storage and transport solutions will contribute to ensure the stability, reliability and flexibility of the energy 10 systems in that context. Ammonia is a well-known chemical of formula NH3 and, amongst other electrofuels, 11 a promising energy carrier and carbon-free combustible fuel. In the present experimental study, engine per-12 formance, combustion characteristics and pollutant emissions of a recent spark ignition engine fueled with 13 premixed ammonia/hydrogen/air mixtures were assessed. Gaseous ammonia blends in a wide range of hydro-14 gen fuel fractions and equivalence ratios were tested at two different engine loads. Results show performances 15 comparable with conventional fuel operation when the appropriate promotion strategies are used. Specifically, 16 blending up to 20% hydrogen in the fuel by volume improves the cyclic stability and avoids misfires, while 17 granting the best work output and indicated efficiencies near stoichiometry. Higher hydrogen fractions result 18 in depleted efficiency, attributed to higher wall heat losses. The combustion duration is directly correlated to 19 the LBV of the mixtures, thus being accelerated by hydrogen blending. The accelerating effect of hydrogen is 20 particularly remarkable during the initial stage of the combustion. Hydrogen appears therefore mainly as an 21 ignition promoter. Increasing the engine load improves the furnished work and allows to extend the operating 22 boundaries in terms of mixture composition. 23