RuRh and RuPt bimetallic dendrimer-encapsulated nanoparticles (DENs) were successfully prepared by a two-step process involving co-complexation, using ruthenium and rhodium or ruthenium and platinum complex ions with partially quaternized fifth-generation poly(amidoamine) (G5-Q), and then coreduction with NaBH4. UV–vis, transmission electron microscope, and laser scattering analyses were used to characterize the bimetallic alloy DENs. The regioselective synthesis of multifunctional hybrid poly(methylhydro)siloxane was achieved by the RuRh and RuPt alloy nanoparticle catalysts. The experimental results showed that the RuRh and RuPt bimetallic DENs presented higher catalytic activity than physical mixtures of their monometallic DENs, respectively. This should be due to synergistic electronic effect of RuRh and RuPt alloy catalysts. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.