Nanometer-sized particles of bismuth are formed by the radiolytic reduction of aqueous solutions of bismuth perchlorate, the size and shape of the particles depending on the nature of the stabilizer. Well-facetted particle shapes (mean diameter : 20 nm) are observed with 2 x 10(-4) M polyethyleneimine or 1 x 10(-3) M polyacrylamide-10 (10% carboxyl groups) as stabilizer, whereas particles of less regular shape (mean diameter : 8 nm) are obtained with 1 x 10(-3) M polyacrylamide-70 (70% carboxyl groups). The reduction of Bi(III) by 1-hydroxy ethyl methyl radicals, (CH3)(2)COH, is accompanied by H-2 production, the bismuth particles formed serving as a catalyst. The absorption spectrum of colloidal bismuth has a weak maximum at 253 nm, which is not rendered by the spectrum calculated from the (rather scarce) dielectric data of Bi. Radiolysis experiments for the preparation of composite metal particles are also reported. The reduction of Cd2+ ions by organic radicals is catalyzed by Bi particles and leads to a cadmium shell. Reduction of Bi(III) in the presence of colloidal silver leads to a blue-shift of the silver plasmon absorption band, which is explained by a deposition of bismuth on the silver particles.