Supercritical deposition is an alternative and promising way to prepare supported metal nanoparticles. Such materials have potential applications in optical, electronic, magnetic devices and in catalysis. Supercritical deposition involves the dissolution of an organometallic precursor (OM) in a supercritical fluid and the exposure of a porous support to the solution. After adsorption of the precursor on the support, the metallic precursor is converted to its metal form by chemical or thermal reduction. This promising catalyst preparation technique results in small particle sizes and homogeneous dispersions. An additional advantage of this technique is the ability to thermodynamically control the metal loading. Even though the technique has been used to prepare a wide variety of supported single metal nanoparticles, there are hardly any studies on application of supercritical deposition to preparation of binary metal alloy nanoparticles. In this study, the preparation Pt-Pd binary metal alloys nanoparticles on a carbon support (Black Pearle 2000) by the supercritical CO2 deposition method was investigated. The carbon supports were first impregnated with dimethyl (cyclooctadiene) platinum (PtCODMe2) and palladium acetylacetonate (Pd(acac)2) from supercritical CO2 solutions. The resulting composites were converted to Pt-Pd/C by chemical reduction in supercritical CO2 with hydrogen. The catalysts were characterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-Ray spectroscopy (EDXS). Thermodynamics and kinetics of adsorption of Pd(acac)2 on BP2000 support was also investigated. The adsorption isotherm was measured at 3000 psi and 60 0C and was represented by a linear model. The kinetics of adsorption of was well represented by a model consisting of pore diffusion and local equilibrium in the pores. The factors influencing the formation of Pt-Pd binary nanoparticles including reduction temperature, precursor loading and hydrogen concentration were investigated. The particle size of the alloy particles ranged from 2 to 10 nm and were found to increase with increasing reduction temperature and hydrogen concentration. The composition of the alloy on the surface could be controlled using the adsorption isotherms.