The cobalt(III) complex of a Schiff base diquinone ligand Co-III(Cat-N-BQ)(Cat-N-SQ) undergoes an entropy-driven valence tautomeric equilibrium in solution yielding the Co-II(Cat-N-BQ)(2) species. This suggestion is supported by variable temperature electronic and H-1 NMR spectra, as well as by solution magnetic susceptibility data. The calculated thermodynamic quantities associated to the redox equilibrium are Delta H = 42.0(0.8) kJ mol(-1) and Delta S = 140 (3) J mol(-1) K-1.