Noble metals (Pt, Pd, and Ph) supported on Al2O3, K2O/Al2O3, CeO2/Al2O3, and K2O/CeO2/Al2O3 were prepared and characterized with respect to surface area, pore volume, and temperature-programmed desorption of CO2. The effects of K2O on the noble-metal catalysts for carbon monoxide and hydrocarbon oxidation were investigated. The reactions were carried out under the stoichiometric and oxygen-deficient conditions. Under the stoichiometric point, the Pd-containing catalysts exhibit higher activity than the Pt-containing catalysts for both CO and C3H6 oxidation. Moreover, Pd/K2O/CeO2/Al2O3 is the most active catalyst among the powder catalysts in this study. Under the oxygen-deficient conditions and in the presence of water, the CO conversions on Pd/Al2O3 and Pd/CeO2/Al2O3 are significantly lower than those on Pt/Al2O3 and Pt/CeO2/Al2O3, respectively. In contrast, the Pd-containing catalysts exhibit higher C3H6 conversion than the Pt-containing catalysts. However, the CO conversions on the Pd-containing catalysts can be promoted by the addition of K2O. On the other hand, the test results of the monolithic catalysts revealed that the CO conversion on PdRh/K2O/Al2O3-CeO2 is quite close to that on PtRh/Al2O3-CeO2 under the simulative gases and the ECE-40 mode driving cycle test. PtRh/Al2O3-CeO2 is the typical composition of catalytic converters for two-stroke motorcycles. It infers that PdRh/K2O/Al2O3-CeO2 is a promising catalytic converter for a two-stroke motorcycle.