The catalytic combustion of carbon black (CB), a model compound for soot, was studied in the presence of MgO and of KOH/MgO, the latter prepared by impregnation of MgO with an aqueous solution of KOH. The catalysts were characterized by atomic absorption spectroscopy, thermal programmed desorption, carbothermic reduction, X-ray photoelectron spectroscopy, diffuse reflectance FIF-infrared spectroscopy and X-ray diffraction. The addition of potassium to MgO shows significant catalytic activity for CB combustion. The role of potassium can be attributed to: (i) an important increase of oxygen concentration on the surface of the catalyst, (ii) an interaction between K and MgO which leads to a weakening of the Mg-O bonds, thus facilitating the formation and migration of oxygen species on the surface; this migration might be a rate-limiting step in the combustion and (iii) an easier decomposition of carbonates which takes place at lower temperature than on MgO. Furthermore, the KOH/MgO catalyst showed a high stability, as its activity was kept constant during isothermic cycles of CB combustion in air. (c) 2005 Elsevier B.V. All rights reserved.