In the presence of OH- anions, electrochem. redox reactions of cobalt tetraphenylporphyrin ((TPP)Co) were investigated in dichloroethane solution by thin-layer cyclic voltammetry and spectroelectrochem. In the presence of OH-, OH- was axially coordinated to (TPP)CoII to generate (TPP)CoII(OH)-. This complex was oxidized to (TPP)CoIII(OH) at Eo' = 0.53 V (vs. SCE). The Eo' for the first porphyrin-ring oxidation was neg. shifted to 0.88 V. When the amount of added OH- anions was more than two equivalent amount, no second OH- coordination reaction was found. Electrochem. oxidation reaction of (TPP)CoII(OH)- generated (TPP)CoIII(OH)21- species. The half-wave potential of this reaction was shifted neg. with increasing OH- concentration The first and second porphyrin-ring oxidns. occurred at Eo' of 0.57 V and 1.07 V, resp. A chem. reaction following the second oxidation step was observed and a reversible redox step of the reaction product was observed at 1.32 V. Mechanism for the coordination and electrochem. reactions of (TPP)Co in the OH- titration process are proposed. Stability constants for various complexes are calculated

Key words: REACTION MECHANISM, COBALT COMPLEX, ELECTROCHEMICAL REACTION, PORPHYRIN, COMPLEX REACTION, ELECTRODE REACTION, HYDROXIDE RADICAL