Catalytic oxidation of hydrocarbons in lean-burn natural-gas engine exhaust has been studied for Pt and Pd supported on alumina. A Pt-Pd/alumina catalyst exhibited higher and longer-lasting hydrocarbon oxidation activity than Pt-Rh/alumina, Pt/ alumina, and Pd/alumina catalysts. Increasing the palladium content in Pt-Pd/alumina catalyst increased the oxidation activity and had more durability. While increasing the platinum content a little bit also improved the activity, adding much more did not. Supporting the platinum on alumina retarded the sintering of Pd and PdO, thus lengthening the oxidation activity of the Pt-Pd/alumina catalyst.