Colloidal copper particles (20-100 nm) are Formed in the gamma-irradiation of aqueous solutions of KCu(CN)(2); which also contain methanol or 2-propanol as OH scavenger. The radiation chemical yield is of the order of 0.1 Cu atoms formed per 100 eV absorbed radiation energy and decreases with increasing concentration of excess KCN. A reduction mechanism is proposed, in which the hydrated electron reacts with Cu(CN)(2)(-) whereas organic radicals attack,2 copper-I species, possibly colloidal Cu2O, present in low concentration by partial hydrolysis of Cu(CN)(2)(-). Free copper atoms do not appear as intermediates in this mechanism. The optical absorption spectrum of Cu particles of different size is also reported; it contains the plasmon band in the 560-580 nm region and a UV band at 222 nm and becomes flatter with increasing particle size. The copper particles have almost spherical shape when formed at high irradiation dose rates but exhibit very pronounced polyhedra and rodlike contributions at low-dose rates.