The catalytic combustion of acetone and methyl-ethyl-ketone (MEK) has been studied over a manganese oxide, Mn2O3, The reactant conversion has been followed as a function of the reaction temperature and it has been observed that lower temperatures are required for the combustion of acetone than for MEK. The performance of Mn2O3 in the, combustion reactions when alkali and acid ions are added to the oxide has also been investigated. A significant improvement of the catalyst performance is observed when Cs+ and Na+ are used as additives. The data of the ignition curves have been fitted to a simplified model where both ketone combustion reactions are assumed to have power law rate equations, which are first-order with respect to the corresponding organic molecule and do not depend on the oxygen partial pressure. Differences in the apparent activation energy values estimated with this model for acetone and MEK combustion are significant only when acid ions additives are present.