Non-precious metal (NPM) catalysts are currently under development to replace the expensive platinum-based materials currently utilized for oxygen reduction in PEM fuel cells. In this work, systematic studies were carried out to examine the effect of central metal, chelating ligand, and solvent on the O-2 binding activity of a series of M-N-2 and M-N-4 NPM catalysts (Im)MLn where M=Cu2+, Fe2+, Fe3+, Ni2+ and Co2+, L= diaminotriazole or porphyrin, and a support ligand imidazole (Im). O-2 and H2O binding energies were calculated for all the catalysts. Cu2+-based catalysts exhibit no activity toward O-2 regardless of the ligand, Fe2+- and Co2+-based catalysts show the strongest O-2 binding, and the rest fall in between. This is in alignment with the energy gap between the metal 3d(z2) and the in-plane anti-bonding pi* orbital on the O-2 with the larger the energy gap, the weaker the interaction. Porphyrin-based catalysts bind weakly with H2O compared to their diaminotriazole counterparts, which is attributed to the larger energy gap between the HOMO of H2O and the higher lying LUMO of Porphyrin-based catalysts resulted from a stronger anti-bonding interaction between their metal d orbitals and the sigma(N) orbitals of porphyrin. We propose that the initial O-2 absorption activity of M-N-4 or M-N-2 catalysts in the oxygen reduction reaction be considered on the basis of the relative binding of O-2 to H2O. (C) 2012 Elsevier Ltd. All rights reserved.