The redn. of NOx with CO and H2 on Pt-Mo/a-Al2O3 catalysts as an alternative for Pt-Rh catalysts was studied for exhaust gas purifn. in lean-mixed engines in oxidizing range. The NOx content has a significant influence on the catalyst activity. The H2/O2 reaction is complete at .apprx.70 Deg in absence of NOx. A NOx content of 0.1% results in an increase of the NOx decompn. reaching a max. of 80% at .apprx.90-120 Deg and a decrease at higher temps. (16% at .apprx.200 Deg). NOx was not reduced by CO at the given conditions and CO inhibited the NO/H2 and the O2/H2 reaction. The activities of the Pt-Mo-catalysts were compared with other 3-way catalysts contg. 0.2% Pt, 0.7% Mo, and 0.1% Cu, Ni, or Co. A significant increase of the NOx reaction rate was reached by the Co-contg. catalyst at presence of 0.3-0.6% CO in the exhaust gas. [on SciFinder (R)]