The formation process of ultrafine metal particles in water was examined with a cryogenic transmission electron microscope by using a rapid-freezing specimen-preparation technique. the particles were formed by hydrogen reduction in an aqueous solution of chloroplatinic acid with or without a surfactant. The ultrafine platinum particle was obtained with the surfactant, and they concluded as formed not in a micelle as a nucleation site, but in arbitrary places in solution. During the formation process, the particles are coated and stabilized by surfactant molecules so as to not aggregate, when using the surfactant. (C) 2002 Elsevier Science B.V. All rights reserved.