Electrochemical and spectroelectrochemical properties of five cobalt(III) acetate complexes [CoIII3(mu(3)-O)(CH3-CO2)(5)(OR)(py)(3)][PF6] are described, where py = pyridine and R = OCCH3 (A), H (B), CH3 (C), CH2CH=CH2 (D), and CH2C6H5 (E). Each is reduced irreversibly as observed by cyclic voltammetry at room temperature and at -40 degreesC in acetonitrile at scan rates up to 20 V s(-1), but oxidized reversibly to a mixed-valence Co(III)(2)Co(IV) species at similar to1.23 V vs the ferrocenium/ferrocene couple. Controlled potential coulometry confirmed a one-electron-oxidation process. Spectroelectrochemical oxidation of A at 5 degreesC showed isosbestic points in the electronic absorption spectrum that showed the oxidized complex to be stable in solution for at least 1 h.