On the basis of spin-unrestricted hybrid gradient-corrected Becke, Lee, Yang, and Parr B3LYP density functional calculations and the reaction site models Fe(NH2)(2)(NH3)(2) for Fe-II, and Fe(NH2)(2)(NH3)(2)OH for Fe-III, Fe-II is predicted to be the active site for the four-electron reduction of oxygen by heat-treated iron macrocycles. It is favored over Fe-III in the first step of the mechanism because of a site blocking effect: H2O bonds strongly to the Fe-III site, blocking it against O-2 adsorption, and it does not bond strongly to Fe-II. The stronger bonding of the product of the first reduction step, OOH, to Fell compared to Fe-II also helps by contributing to a more positive reversible potential for its formation over Fell. Subsequent reduction steps have high reversible potentials over both centers, paralleling an earlier study of oxygen reduction over a single Pt site. However, the important difference compared to Pt is the hydrogen-bonding interaction between (OHOH) bonded to Fe-II and a nitrogen lone-pair orbital in the N-4 chelate. This is in addition to the O lone-pair donation bond to the Fe-II center and is proposed to prevent hydrogen peroxide from leaving as a two-electron reduction product, as it was predicted to do over a single Pt site, and provides a path for reduction to water.