The electrochemical behavior of cobalt 5,10,15,20-tetrakis(alpha,alpha,alpha,alpha-2-pivalamidophenyl)porphyrin (Co(II)TpivPP) was examined in benzonitrile solutions. The electrochemical responses were changed significantly by the addition of 1-methylimidazole which coordinates once to the Co(II) and twice to the Co(III) porphyrin. The corresponding equilibrium binding constants were estimated as 2.9 x 10(3) M-1 and 1.7 x 10(20);M-2. The cyclic voltammetric responses were affected by the rates of the ligand association/dissociation reactions, Formation of the O-2 adduct of the Co(II) porphyrin resulted in only small changes in the cyclic voltammetric pattern. Comparison between Co(II)TpivPP and cobalt :tetraphenylporphyrin ((CoTPP)-T-II) as catalysts for the electroreduction of O-2 showed the latter to be the superior catalyst both when adsorbed on graphite electrodes and when dissolved in thin layers of acidified benzonitrile where Co(II)TpivPP exhibited unimpressive catalytic activity compared with (CoTPP)-T-II, which appeared to catalyze the reduction of O-2 by significantly more than 2 electrons per molecule.