The influence of the axial organic ligand R on the electrochemical oxidation of the compounds [RCo III(salen)DMF)], where salen is bis(salicylaldehyde)ethylenediimine, and R=CH 3, C 2H 5, n-C 3H 7, n-C 4H 9, s-C 4H 9, i-C 4H 9, CH 2Cl, CF 3CH 2, c-C 6H 11CH 2, c-C 6H 11, C 6H 5, C 6H 5CH 2, p-CH 3C 6H 4CH 2, and p-NO 2C 6H 4CH 2, was studied by means of cyclic voltammetry in dimethylformamide (DMF), 0.2 M in tetraethylammonium perchlorate (TEAP), at 25 and −20°C, with a platinum disc working electrode. The above-mentioned compounds can be classified according to their electrochemical behavior. (a) The complexes with R=CH 3, C 2H 5, n-C 3H 7, n-C 4H 9, c-C 6H 11CH 2, and C 6H 5 undergo a reversible one-electron oxidation in the 10–50 V s −1 potential scan range. At slower scan rates, the oxidized product decomposes chemically. At −20°C, this chemical step is slow, and a reversible one-electron electrochemical oxidation is observed. (b) The compounds with R=CH 2Cl, C 6H 5CH 2, p-CH 3C 6H 4CH 2, and p-NO 2C 6H 4CH 2 undergo a quasi-reversible one-electron oxidation at room temperature. At −20°C, the electrochemical process becomes more complex. A following chemical reaction is coupled to the quasi-reversible one-electron transfer. Two reduction peaks are observed. (c) The compounds with R=i-C 4H 9, s-C 4H 9, and c-C 6H 11 undergo a reversible one-electron oxidation at −20°C. At room temperature, the irreversible chemical reaction following the electron transfer step is too fast to allow the isolation of the electrochemical step. (d) At −20°C, the derivatives with R=C 2H 5, c-C 6H 11-CH 2, and c-C 6H 11 are adsorbed at the electrode surface. Evidence indicates that the reagent in these reactions is the pentacoordinated species [RCo III(salen)]. A linear free-energy relationship between E 1/2 (for reversible processes) and the Taft polar parameters σ * was obtained with a slope of ϱ *=0.25±0.03. As expected, the benzyl derivatives which present mesomeric effects do not fit this polar correlation. The rate of the electrochemical oxidation is also affected by the nature of the ligand R. For the ligands which are strong electron-withdrawing groups and for the benzyl derivatives, the rate of the electrochemical oxidation of the metal ion decreases at room temperature. At lower temperatures, it is suggested that the oxidation to the Co IV-R species is followed by a chemical reaction in which this complex is partly transformed into a Co III(R ·) species, which is reduced at a much more cathodic potential than the Co(IV) species.