We describe in this report the preparation, structural characterization, and phase behaviors exhibited by supported metallic and bimetallic nanoparticles. Homometallic nanoparticles of either Pt or Ru were synthesized by the reduction of various precursors ((CH3)(2)Pt(COD), H2PtCl6, and RuCl3) onto different carbon supports: Vulcan XC-72 (VXC) and Shawinigan Acetylene Black (SAB). The choice of precursor has a large structural influence on the reductive condensation of the Pt metal particles. All of the various precursors and supports produced particles with very similar size distributions, with the exception of (CH3)(2)Pt(COD), which formed a complex distribution of small (20 Angstrom) and large (>50 Angstrom) particles. The centerpiece of this study is the characterization of the growth behaviors seen in the synthesis of binary Pt-Ru nanoparticles. These heterometallic particles were synthesized via a seeded reductive condensation of one metal precursor onto pre-supported nanoparticles of a second metal; the latter serve as nucleating sites for the growth of the binary phase. As shown via data from X-ray photoelectron spectroscopy (XPS), electron microscopy, and energy-dispersive X-ray analysis (EDX), this growth technique yields fully alloyed metallic nanoparticles, albeit ones of varying size and compositional distributions depending on the specific conditions used. Generally we found that the particles had a wide composition distribution. The nature of this distribution and the correlations between the nanoparticle sizes, compositions, and structures embedded in it were characterized in depth by scanning transmission electron microscopy (STEM). These results are used to establish an apparent size correlated binary phase diagram of the bimetallic (Pt-Ru) nanoparticles. The structural properties of the supported bimetallic nanoclusters are different from that of the bulk, as evidenced by the presence of strongly persistent metastable structures that are not found in the bulk phase diagram.