The colloidal dispersions of polymer-protected platinum/rhodium bimetallic cluster particles are easily prepared by refluxing solutions of hexachloroplatinic(IV) acid and rhodium(III) chloride in ethanol/water (1/1 v/v) in the presence of poly(N-vinyl-2-pyrrolidone). These colloidal dispersions of the Pt/Rh bimetallic cluster particles are very stable and have a size distribution from 2 to 7 nm in diameter. The electronic spectra and the transmission electron micrographs suggest that the colloidal dispersions are not a mixture of monometallic Rh and monometallic Pt particles but are mostly composed of Pt/Rh alloy particles. Electron microprobe analysis indicates that the metal composition of each alloy particle with nearly average diameter is kept as the charged mole ratio of Pt/Rh. Extended X-ray absorption fine structure analyses, high-resolution electron microprobe analysis, and scanning tunneling microscopy indicate that colloidal alloy particles consist of an assembly of Pt/Rh bimetallic microclusters, each of which has a Pt core and a modified Pt core structure of colloidal dispersions of Pt/Rh(1/4) and Pt/Rh(1/1 or 4/1) bimetallic clusters, respectively. The importance of the microcluster is discussed in detail for the formation of Pt/Rh bimetallic cluster particles.