We report that by controlling the orientation of the impinging O-2 before it binds to heme, we can lower the activation barrier of O-2 dissociation from the heme-O-2 adduct to a value comparable to the activation barrier of the well-known platinum catalyst, using ab initio calculations based on density functional theory (DFT). The activation barrier for the O-O bond cleavage for the side-on heme-O-2 configuration is lower than that for the end-on heme-O-2 configuration. Our results suggest that heme may be utilized as a cathode electrode catalyst in polymer electrolyte fuel cells (PEFCs). (C) 2004 Elsevier B.V. All rights reserved.