The electrochemical behaviors of a cobalt porphyrin (Co octaethylporphyrin (Co-OEP)) adsorbed on highly oriented pyrolytic graphite (HOPG) were examined. Co-OEP on HOPG exhibits highly reversible voltammograms with a peak separation of almost zero. The redox potential of Co-OEP on HOPG in sulfuric acid was much higher than a previously reported value of Co-OEP on an edge plane electrode in perchloric acid. The high potential would be due to the instability of cationic trivalent state of Co on HOPG; hydrophilic trivalent state is not familiar with a hydrophobic surface of HOPG, and the interaction between cationic molecules causes positive shift in the redox potential. Certain anions (Cl-, Br- and ClO4-) cause a negative shift of midpoint potential of Co-OEP, since they counteract the positive charge of trivalent Co in Co-OEP. The dissociation constants of Co-OEP-CI and Co-OEP-Br were determined from the potential shift. The constants indicate that Br(-)coordinates on Co-OEP more strongly than Cl-. ClO4- has a different effect on the redox behaviors of Co-OEP than halides. While the oxidation of CO oxidation by Co-OEP was suppressed by the coordination of these halides, ClO4- did not significantly decrease the CO oxidation current, but rather decreased the overpotential for CO oxidation. These results suggest that halides coordinate on trivalent Co-OEP, while ClO4- interacts with positive charge of trivalent Co-OEP without coordination. (C) 2014 Elsevier Ltd. All rights reserved.