Intermetallic compounds (IMCs) have unique geometric and electronic structure compared with their component pure metals. We already found that PtCu IMC supported on silica gel was more active than Pt/SiO2 for the preferential oxidation (PROX) of CO in excess hydrogen. In this study, we have tried to prepare these IMC particles on alumina by the liquid-phase reductive deposition (LRD) method, characterized them by X-ray diffraction (XRD), transmission electron micrograph (TEM), and Fourier transform infrared (FT-IR) spectroscopy, and carried out the PROX reaction with a continuous flow reactor. PtCu IMC was formed as single-phase particles on alumina with the particle size of 3-5 nm. PtCu/Al2O3 gave much higher activity for the PROX reaction than PtCu/SiO2 and Pt/Al2O3 prepared by the LRD method and Pt-Cu/Al2O3 prepared by the usual impregnation method, indicating the superiority of alumina support, IMC formation, and the LRD method. In situ FT-IR spectroscopy and transient reaction measurements suggest that during the PROX reaction on PtCu/Al2O 3, CO and O2 are adsorbed preferentially on Pt and Cu atoms, respectively, and that the formation of CO2 proceeds through bicarbonate intermediate species with the help of hydroxyl groups of alumina.