The equilibrium adsorption amount of Co porphyrins possessing various meso-substituents on Vulcan XC 72R was determined. The adsorption amount differs significantly among the Co porphyrins, and the difference can be clearly explained by the bulkiness and orientation of the meso-substituents. These results indicate the existence of a specific interaction between the porphyrin and carbon black. The electrocatalytic O-2 reduction activity by Vulcan XC 72R-supported Co porphyrins was also determined. A significant difference in the onset potential was observed. A comparison with the results of Co porphyrins on glassy carbon as well as the redox potential of Co porphyrins in an organic solvent revealed that the porphyrin Vulcan interaction affects the onset potential of the electrocatalytic 0, reduction by Co porphyrins. (C) 2004 Elsevier B.V. All rights reserved.