A Cu-I complex of 3-ethynyl-phenanthroline covalently immobilized onto an azide-modified glassy carbon surface is an active electrocatalyst for the four-electron (4-e) reduction of O-2 to H2O. The rate of O-2 reduction is second-order in Cu coverage at moderate overpotential, suggesting that two Cu-I species are necessary for efficient 4-e reduction of O-2. Mechanisms for O-2 reduction are proposed that are consistent with the observations for this covalently immobilized system and previously reported results for a similar physisorbed Cu-I system.