The effect of support on the noble metal catalysts for carbon monoxide and hydrocarbon oxidation was investigated. The reactions were performed under the stoichiometric and oxygen-deficient conditions. Under the stoichiometric point, the activities of the powder catalysts for CO and C3H6 oxidation are in the order Pt/K2O/Al2O3 > Pt/Al2O3 > Pt/Al2O3-SiO2, that is, the same as the order of basicity of these catalysts. Under an oxygen-deficient condition, Pt supported on an acidic support, Al2O3-SiO2, exhibits the higher C3H6 conversion and the higher activity for the steam re-forming reaction. Ln contrast, Pt supported on a basic support, K2O/Al2O3, exhibits the higher CO conversion and the higher activity for the water-gas shift reaction, The order of activity of the powder catalysts for the water-gas shift reaction is the same as the order of basicity of these catalysts. On the other hand, the testing results of the monolithic PtRh-containing catalysts by the simulative gases and the ECE-40 mode driving cycle also reveal the same trend as that of the Pt powder catalysts. Furthermore, the addition of K2O on PtRh/Al2O3-CeO2 not only increases the basicity of the catalyst but also significantly reduces the CO emission under the ECE-40 mode driving cycle test. Therefore, K2O could be a promising additive to a catalytic converter of a two-stroke motorcycle since it can significantly enhance CO conversion.