Spherical Au-Pt alloy nanoparticles of 10 nm average size have been prepared on a H-terminated Si(100) substrate by an extremely simple method of dip-coating. X-ray photoelectron spectroscopy and glancing-incidence X-ray diffraction confirm the formation of Au-Pt alloy. The Au3+ ions are first reduced on the Si substrate upon dipping, and the freshly formed Au nuclei then work as a "catalyst" by promoting the reduction of PtCl62- ions on the Au nuclei. The subsequent interdiffusion of Au and Pt atoms leads to the observed alloy formation. The present method provides an environment-friendly, low-cost route to preparing anode electrodes in fuel cells.