The hydrocarbon-assisted, selective catalytic reduction of NOx constituents in lean-bum engine exhaust is benefited by passage of the exhaust over a staged catalyst bed comprising a first stage of an alumina-supported silver catalyst for oxidation of NO to NO.sub.2 and partial oxidation of added hydrocarbon to an aldehyde. Downstream of the silver catalyst is a second catalyst selected for the reaction of hydrocarbon and aldehyde species with NO.sub.2 to reduce it to nitrogen. Gamma-alumina and BaY zeolite are examples of suitable second stage catalysts.