Abstract: The exhaust gas discharged from an engine such as an internal combustion engine or the like is treated to convert harmful or noxious pollutant components such as nitrogen oxides, carbon monoxide and unburned hydrocarbons to innocuous compounds, by contacting the exhaust gas with two stages in series of a catalyst composition containing 8% to 20% by weight of copper as equivalent copper oxide, 1% to 5% by weight of cobalt as equivalent cobalt oxide, and 0.5% to 3% by weight of manganese as equivalent manganese dioxide, deposited on particles of transitional alumina having a surface area in the range of 200 to 400 square meters per gram. The copper and cobalt are at least partially present in the form of reduced oxides, and the catalyst may also contain 1% to 5% nickel as equivalent nickel oxide.