Selective catalytic reduction of nitrogen monoxide (NO) by hydrocarbon in an oxidizing atmosphere has been studied over platinum-MFI zeolite (Pt-MFI) in synthesized or actual diesel engine exhaust gases. The activity of Pt-MFI in the synthesized gas, containing 10% water, changed in the early stage of the use, leveled off after 150-200 h, and remained constant for more than 800 h. The Pt-MFI catalyst also showed stable activity at 423-773 K and 10 000-150 000 h(-1) (gas hourly space velocity) in actual engine exhaust with light oil as a fuel. The degree of nitrogen monoxide reduction increased linearly upon addition of ethylene into the exhaust gas.