High resolution electron microscopy and X-ray microchemical analysis were used to characterize the composition, size, distribution, and morphology of Pt-Ru particles with nominal Pt:Ru ratios of 1:1 and 3:1, supported on carbon black. The particles are predominantly single nanocrystals with diameters in the order of 2.0 to 2.5 nm. Occasionally, twinned particles are also observed. All investigated particles represent solid solutions of Pt and Ru with compositions very close to the nominal one. Based on two-dimensional projection in high resolution images, it is suggested that the well-resolved particles are of cubooctahedral shape. In addition to {200} and {111} facets, {113} facets are also observed.