Counterion size effects on the conproportionation constants K-c1 and K-c2 of the windmill complex [Cp3Co3(S6C6)] (1) (Cp = eta(5)-C5H5) in the mixed-valence states 1(-) and 1(2-) respectively have been investigated electrochemically in solutions of alkaline metal ions and crown ether. Addition of 18-crown-6 (18C6) or 15-crown-5 (15C5) to a solution of 1 in NaBPh4 + THF brings enormous increases in both K-c1 and K-c2 at a concentration similar to that of NaBP4, and the change of K-c2 is more significant than that of K-c1. This behaviour can be simulated using the complexation constants of crown ether with a cation obtained by conductivity analysis and several equations concerning association, ion pairing equilibrium, conproportionation and charge balance assuming that most of 1(-), 1(2-) and 1(3-) exist in the form of ion pairs.