The voltammetric characteristics of the title adducts [Co(Cp) (S(2)C(2)X(2))L] (X =CN, CF3, COOCH3, 4-pyridyl, phenyl; L= tributylphosphane or tributyl phosphite) in acetonitrile solutions are reported. Both the phosphane and the phosphite adducts were oxidized and reduced through one-electron transfer processes followed by homogeneous chemical reactions for all X except phenyl; the oxidized adduct dissociated into the free complexes [Co-III (Cp) (S(2)C(2)X(2))] and the cation radical L(+.), and the reduced adduct rapidly dissociated into the reduced form of the free complexes [Co-II(Cp) (S(2)C(2)X(2))](-) and L. When X = phenyl, the oxidation was similar but the reduction was preceded by the dissociation of L. The oxidation and reduction half-wave potentials of the adducts were more negative than those of the corresponding free complexes, and they were linearly dependent on the adduct formation constant.