Bimetallic nanoparticles of I'd and Pt at various molar ratios have been prepared by the coreduction of H2PdCl4 and H2PtCl6 with hydrazine in water/AOT/isooctane microemulsions at 25 degreesC. The size, structure, and composition of the resultant nanoparticles were characterized by TEM, XRD, EDX, and XPS. It was shown that the bimetallic nanoparticles obtained at feeding compositions of [H2PdCl4]/[H2PtCl6] = 9/1 similar to 1/9 were essentially monodispersed and that their mean diameters with a positive deviation were in the range of 7 similar to 11 mn. The bimetallic nanoparticles had a homogeneous alloy structure except when the molar ratio of Pd/Pt was above 9/1. Also, the composition for each particle was roughly consistent with that of the feeding solution. Furthermore, the characterization and formation kinetics of the particles suggested that the reduction of PtCl6- and PdCl4-2 ions was finished before the formation of nuclei , the nucleation rate of Pd was much faster than that of Pt, the size of bimetallic nanoparticles was determined by the number of the nuclei formed at the beginning of reaction, and the nuclei for a bimetallic system might be composed of I'd and Pt. According to these, a formation process for Pd/Pt bimetallic nanoparticles was suggested.