The reactivity of a Mn-substituted hexaaluminate catalyst in the catalytic combustion of gasified biomasses-derived fuels is addressed. The combustion activity of the main fuel components, i.e., CO, H-2, C2H4 and CH4, is investigated, along with that of NH3 that is present in the biomasses-derived fuels. It is found that significant amounts of NOx are formed from ammonia during the catalytic combustion process, but the occurrence of gas-phase homogeneous reactions involving NH3 and NO in the presence of oxygen strongly reduce NOx formation at high temperatures.