UV−visible and Raman spectroscopies as well as electrochemical techniques have been used to characterize cis- and trans-[Co(III)(en)2Cl2]Cl (en = ethylenediamine) complexes and the γ-alumina-supported cis-Co(III) complex. It is shown that the electrochemical reduction of these complexes occurs according to a multistage mechanism involving two electrochemical steps, with the formation of a dimer that was characterized by UV−visible spectroscopy (intervalence band at 670 nm). The apparent standard redox potential for each step has been determined, and experimental results reveal that cis and trans complexes present similar electrochemical characteristics. It is also shown that the deposition of trans-[Co(III)(en)2Cl2]+ on γ-alumina leads to an inner-sphere complex (ISC) in a cis configuration in which Cl- ligands are substituted by OH or O- surface groups of alumina. These changes in the coordination sphere of the complex induce a substantial decrease of its apparent redox potential since it is −0.63 V/SCE (saturated calomel electrode) for the γ-alumina-supported cis-Co(III) complex, whereas values of −0.17 and −0.35 V/SCE were determined in dimethyl sulfoxide (DMSO) for the trans and cis precursor complexes, respectively.