MgO, CaO, an equal ratio mixture of the oxides (50-CaO) and a sample with 10% potassium added to the previous mixture, showed increasing activity for the catalytic combustion of carbon black, a model compound for soot. These catalysts were characterized by XPS, BET, DRIFTS and AAS. Activity was measured by TGA and by TPO; CO and CO2 concentration in the outlet gas flow were measured by NDIR. Pure MgO was barely active. The 50-CaO catalyst was as active as the pure CaO in spite of a lower content of calcium. This was attributed to greater contact area and better dispersion of calcium, lesser bulk and surface carbonation, cooperative effect of magnesium that diminishes the stability of adsorbed surface compounds, and a greater accessibility of O2(g) to calcium sites. Potassium introduction over the 50-CaO produced high activity, related to a increase of the surface concentration of oxygen on the catalyst and larger CO2/CO ratio in the exit gases. Moreover, potassium addition resulted in a lower content of calcium on the surface, hence less carbonation of the catalyst