Colloidal tin is formed in gamma-irradiated solutions of SnCl2, which also contain 0.5 M propanol-2 and 5 x 10(-4) M polyethyleneimine. The reduction occurs in the first stages of irradiation by hydrated electrons. In the later stages, when some colloidal particles have been formed, the (1-hydroxymethyl)ethyl radicals, (CH3)(2)COH, which as well as the hydrated electrons are primary radiolysis products, contribute to the reduction. The absorption spectrum of Sn particles, which are essentially smaller than the wavelengths of light, is reported. In the presence of colloidal gold particles, the reduction of Sn(II) by organic radicals is strongly enhanced. The effect is ascribed to the cathodic polarization of the Au particles by electron transfer from the radicals and subsequent reduction of Sn(II) directly on the surface of the Au particles. The optical changes accompanying Sn deposition are also described. For small deposits, the plasmon band of gold is strongly damped and only part of the deposited tin can be reoxidized by oxygen. When larger amounts of tin are deposited, alloying occurs to a great extent.