The reductive condensation of a carbon-supported molecular cluster precursor, PtRu5C(CO)is, into a bimetallic nanoparticle has been followed by using in situ extended X-ray absorption fine structure spectroscopy, temperature-programmed desorption, and scanning transmission electron microscopy. The data reveal that during activation in hydrogen the metal centers associated with the molecular precursor lose the stabilizing CO shell and assume. an increasingly metallic electronic character. This support-mediated condensation process is highly activated. The incipient Pt-Ru nanoparticles initially form a disordered structure at 473 K in which Pr is found preferentially at the core of the condensing particle. After further high-temperature treatment to 673 K, the nanoparticles adopt an inverted structure in which Pt appears preferentially at the surface of the equilibrated bimetallic nanoparticle.