The kinetics of the spontaneous reduction of hexanitrocobaltate(III) ion have been investigated spectrophotometrically (λ=360 nm at 298 K) in ethanol or acetone water mixtures containing 0, 10, 25 and 50 % of organic solvents. The mechanism of this process was expressed by means of two consecutive first-order reactions. Both of the first-order rate constants decrease with the increasing nonaqueous species content. The reduction of hexanitrocobaltate(III) ion in the mixtures was much slower than the corresponding reaction in water.