Colloidal dispersions of rhodium (Rh) nanoparticles have been synthesized by the reduction of Rh ions (III) in high-temperature and high-pressure water, ethanol, or water-ethanol mixture under the existence of the protective polymer of poly(N-vinyl-2-pyrrolidone). The possibility of the regulation of the particle size and size distribution has been tested under several solvents at various temperatures and pressures. At 473 K and 25 MPa, particularly, concentrated colloidal dispersions of Rh particles of 2.5 +/- 0.5 nm were synthesized from the ionic solution of ethanol ([Rh] = 15 mM) within a few seconds. Dilute colloidal dispersions of Rh particles were also synthesized from the, dilute ionic solution ([Rh] = 1.5 mM) with a diameter of 2.0 +/- 0.4 nm. From the water solution, Rh particles tended to form aggregates, especially for the lower concentration solution. In the case of solutions in water and ethanol mixture, the average diameter of Rh particles tended to be larger than in ethanol solution, and their distribution became broad. (c) 2005 Elsevier Inc. All rights reserved.