Solutions containing KAu(CN)(2) (similar to 5 x 10(-4) M), methanol (0.3 M), and nitrous oxide (2.5 x 10(-2) M) are gamma-irradiated in the presence of colloidal gold(similar to 6 x 10(-5) M; mean particle size, 15 nm). The hydroxymethyl radicals,. CH2OH, which are generated in these solutions, reduce Au(I) in Au(CN)(2)(-), and the reduced gold is completely deposited on the gold seeds to yield larger particles. The particle growth is followed spectrophotometrically and by electron microscopy. A mechanism is proposed in which the radicals transfer electrons to the gold particles and Au(CN)(2)(-) is subsequently reduced by the stored electrons directly at the surface of the particles. In further steps of particle enlargement, Au(CN)(2)(-) is reduced in solutions in which the gold particles synthesized in the preceding step serve as seeds, the result being larger and larger gold particles up to 120 nm. The reduction yield is discussed with respect to side reactions of the radicals, such as mutual deactivation and gold-catalyzed H-2 formation. The radiation chemical method makes it possible to enlarge gold particles to any desired size. The reduction of Au(CN)(2)(-) in the absence of seeds is also described.