Supported nanoparticles of platinum on a wide variety of substrates, including carbon aerogel, carbon black, silica aerogel, silica, gamma-alumina, and Nafion 112 film, were synthesized via a supercritical fluid route. The porous substrates and Nafion film were impregnated with an organometallic precursor, dimethyl(1,5-cyclooetadiene)platinum(II) (PtMe2COD), from a supercritical carbon dioxide (ScCO2) solution at 80 degrees C and 27.6 MPa. After depressurization, the impregnated organometallic precursor was reduced to elemental platinum by heat treatment in the presence of nitrogen gas. The resulting nanocomposites were investigated by transmission electron microscopy (TEM), which revealed uniformly dispersed platinum particles on each of the substrates with average particle sizes ranging from 1.2 to 6.4 nm and a narrow particle size distribution. A comparison of nanocomposites produced under different conditions showed that both the metal contents and the particle sizes are controllable.