Potassium-supported Mg-Al hydrotalcite mixed oxides were studied for soot combustion with O-2. The significant activity was elucidated by an oxygen spillover mechanism. First, the surface-activated oxygen on K sites might spill over to the free carbon sites on soot to form a carbon-oxygen complex, ketene group, which was identified as the reaction intermediate. Then the ketene group combined with another active oxygen species to give out CO2. Two kinds of K species, Mg(Al)-O-K (tightly bound to Mg or Al) and free (isolated) K, were determined to be catalytically active sites by kinetic investigations. They could increase the reactivity and amount of surface active oxygen responsible for formation of the ketene group. The stability of K was greatly improved through the interaction with Al, which is a contribution to soot combustion of Al addition into MgO to form a K-supported composite oxide via the hydrotalcite route. (C) 2010 Elsevier Inc. All rights reserved.