Catalysts are disclosed which have good activity and life when used to promote the conversion of hydrocarbons, carbon monoxide and nitrogen oxides contained in small concentrations in gaseous streams, to obtain the less noxious materials, carbon dioxide, water and nitrogen. The catalysts contain a catalytically-promoting, platinum group metal component, a catalytically-promoting, base metal component added as a solid material, and an alumina support component, deposited on a macrosize carrier. The base metal component contains a metal selected from the group consisting of metals having an atomic number from 25 to 28 and rhenium. The base metal and alumina components are preferably composited before deposition on the macrosize carrier, and the composite may also contain the platinum group metal component before such deposition. The catalytically-promoting metal components and alumina support can be comminuted as a slurry, and the liquid medium of the slurry is preferably acidic. The macrosize carrier, bearing the catalytic components may be dried and, if desired, calcined. The catalysts contain platinum or palladium, especially platinum, and preferably, one or more of rhodium, ruthenium and iridium, especially rhodium, in order to promote adequately the simultaneous conversion of hydrocarbons, carbon monoxide and nitrogen oxides to less noxious materials.