The selective reduction of NO_x on various PtMoX-catalysts (X: Li, Na, K, Mg and Ca) loaded on metal oxides such as SiO₂, Al₂O₃, TiO₂, ZSM-5 and silicalite was studied in simulated exhaust gases from an automotive lean-burn engine and a stoichiometric engine and compared with that on PtMo/SiO₂, PtNa/SiO₂, and Pt/SiO₂ catalysts. The temperature window for the selective reduction of NO_x on the PtMoLi-, PtMoNa- and PtMoK-catalysts loaded on SiO₂ was found to be wider and to shift to a higher temperature than that on the other catalysts under lean static conditions. The redox ratio window, in which three-way activity occurred on a PtMoNa catalyst loaded on SiO₂, was also found to be wider than that on PtMo/SiO₂, catalysts and Pt/SiO₂ around the stoichiometric point. On the other hand, XPS, IR and CO adsorption data indicated that the oxidation of Pt on PtMoNa/SiO₂ catalysts was depressed by the added Mo and Na even under excess oxygen conditions, so that the reaction characteristics of the PtMoNa/SiO₂ catalysts were improved.