This paper evaluates the potential of hydrogen (H-2) and ammonia (NH3) as carbon-free fuels. The combustion characteristics and NOx formation in the combustion of H-2 and NH3 at different air-fuel equivalence ratios and initial H-2 concentrations in the fuel gas were experimentally studied. NH3 burning velocity improved because of increased amounts of H-2 atom in flame with the addition of H-2. NH3 burning velocity could be moderately improved and could be applied to the commercial gas engine together with H-2 as fuels. H-2 has an accelerant role in H-2-NH3-air combustion, whereas NH3 has a major effect on the maximum burning velocity of H-2-NH3-air. In addition, fuel-NOx has a dominant role and thermal-NOx has a negligible role in H-2-NH3-air combustion. Thermal-NOx decreases in H-2-NH3-air combustion compared with pure H-2-air combustion. NOx concentration reaches its maximum at stoichiometric combustion. Furthermore, H-2 is detected at an air-fuel equivalence ratio of 1.00 for the decomposition of NH3 in flame. Hence, the stoichiometric combustion of H-2 and NH3 should be carefully considered in the practical utilization of H-2 and NH3 as fuels. H-2 as fuel for improving burning performance with moderate burning velocity and NOx emission enables the utilization of H-2 and NH3 as promising fuels. Copyright (C) 2014 John Wiley & Sons, Ltd.