Quantum chemical predictions are made of reversible potentials for intermediates formed during the four-electron reduction of O-2 by copper laccase, using three loosely associated partially ligated Cu-I centers as models for the active site. Two mechanisms are studied, the platinum surface mechanism in acid electrolyte, and the proposed laccase mechanism for basic electrolyte. Both are found viable. The Cu-II-OH- reduction to H2O step has the lowest potential, making it responsible for the predicted low overpotential. Compared to laccase, the overpotential is somewhat high but completing the ligand coordination would be expected to lead to smaller predicted overpotentials. (c) 2005 The Electrochemical Society.