Using a combination of theory and experiments, we show that ZnO substitutionally doped with Ti or Al oxidizes CO by several parallel reaction pathways that differ from the traditional Mars-van Krevelen (MVK) mechanism. in one, the dopant atoms at the surface of the doped oxide adsorb and activate O-2 so that it reacts with CO. In the other, a surface oxygen atom from the lattice next to the dopant, D, moves onto the dopant and creates an O-D group and an oxygen vacancy. The O-D group is capable of oxidizing a reductant. To test these predictions made by theory, we have prepared Ti- and M-doped ZnO and have shown that these compounds oxidize CO at temperatures at which pure-phase ZnO, Al2O3, or TiO2 do not. The proposed mechanisms were made plausible by studying CO oxidation with O-18(2). (C) 2009 Elsevier Inc. All rights reserved.