We describe the preparation and structural characterization of carbon-supported Pt-Ru nanoparticles with exceptionally narrow size and compositional distributions. The supported bimetallic particles are obtained by reduction of the neutral molecular carbonyl cluster precursor PtRu5C(CO)(16) with hydrogen. A detailed structural model of the nanoparticles has been deduced on the basis of studies by in situ extended X-ray absorption fine structure spectroscopy (EXAFS), scanning transmission electron microscopy, microprobe energy-dispersive X-ray analysis, and electron microdiffraction. These experiments show that the bimetallic nanoparticles have a Pt:Ru composition of 1:5 and an average diameter of ca. 1.5 nm and adopt a face-centered cubic closest packing structure. These results demonstrate a marked sensitivity of the metal particle structure to nanoscale size effects inasmuch as the thermodynamically stable phase for bulk alloys of this composition is hexagonal close-packed. The local metal coordination environment, revealed by multiple scattering analysis of the EXAFS data, shows the presence of a nonstatistical distribution of different metal atoms in the nanoparticles. Specifically, Pt shows a marked preference for segregation to the particle surfaces under an ambient H-2 atmosphere. Oxidation of the alloy particle in O-2 produces an outer metal oxide layer surrounding a metal-only core. This oxidation is easily reversed by exposing the nanoparticles to H? at room temperature.