The radiation-chemical reduction of Co2+ ions in aqueous solution of Co(ClO4)(2) and sodium formate is described. Stable metal sols containing spherical particles with a diameter of 2-4 nm are formed under gamma-irradiation in the presence of polyacrylate as a stabilizing additive. Their optical absorption increases smoothly in the UV region without a maximum up to 200 nm (epsilon(200) = 1.3 x 10(4) M-1 cm(-1)). Pulse radiolysis is used to study the reduction of Co2+; the subsequent formation of colloidal Co occurs via an autocatalytic mechanism. The electrochemical potentials of Co2+/Co-0 and Co+/Co-0 are determined to be -2.3 and -2.9 V, respectively.