A recently developed, first-principles approach that can explicitly consider the influence of the complex solution phase environment as well as the electrochemical potential on electrocatalytic reaction systems was employed. The potential-dependent reaction energies and activation barriers for a detailed sequence of elementary steps that comprise different speculated oxygen reduction reaction (ORR) mechanisms were studied over platinum. The first complete analysis of the potential dependence of the reaction energetics of O2 electro-reduction over Pt(111) is presented. This provides essential insight into the mechanistic factors that dictate the performance of ORR cathodes for use in PEM fuel cells. Periodic gradient-corrected density functional theoretical calculations were used to follow the catalytic reaction pathways and calculate the corresponding energies. A detailed reaction path analysis indicated that the ability of the catalyst to break the O-O bond should not directly relate to the ORR performance. The reaction energy of the initial reduction step was a key factor in dictating catalyst performance, however, consideration of a series of alloy catalysts indicated that this reaction did not become more exothermic on surfaces that more strongly bind atomic oxygen. This is an abstract of a paper presented at the 2007 AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).