Refluxing mixed solutions of palladium(II) chloride and hexachloroplatinic(IV) acid in ethanol–water (1 : 1, v/v) in the presence of poly(N-vinyl-2-pyrrolidone) results in well dispersed and stable colloidal dispersions of polymer-protected palladium/platinum bimetallic clusters. The UV–VIS absorption spectra and transmission electron micrographs indicate that these bimetallic cluster particles are homogeneous in size, each particle containing both palladium and platinum atoms. Dispersions of these bimetallic clusters were used as catalysts for the selective hydrogenation of cycloocta-1,3-diene to cyclooctene at 30 °C under hydrogen gas at atmospheric pressure. The catalytic activity for the partial hydrogenation was found to depend on the metal composition of the particles. A bimetallic cluster with a mole ratio of Pd : Pt = 4 : 1 was the most active catalyst, twice as active as a typical colloidal palladium catalyst, for the diene hydrogenation. The dependence of the activity and the selectivity on the surface structure of the cluster particle strongly indicates that the cluster particle with the highest catalytic activity had a ‘Pd-surrounded Pt core’ structure.