Equilibria between Co-III(bpy)(3,5-DBSQ)(3,5-DBCat) and Co-II(bpy)(3,5-DBSQ)(2) redox isomers have been studied in toluene solution and in the solid state. Enthalpy changes are nearly the same in both media, and Delta H is determined by changes in Co-O,N bond energies with the transition from low-spin Co(III) to high-spin Co(II). Temperature dependence of equilibria is associated with a large entropy change, resulting primarily from low-frequency shifts in vibrational modes with metal reduction.