kinetic and mechanistic studies of the electrocatalytic reduction of o2 to h2o with mononuclear cu complexes of substituted 110phenanthrolines mononuclear cu complexes with a 110phenanthrolinebased ligand adsorbed onto an edgeplane graphite electrode act as electrocatalysts for the 4electron reduction of o2 to h2o a mechanism is proposed for the electrocatalytic o2 reduction that accounts for the observed redox and kinetic dependences on coordinating anions and proton donors in the buffer systematic increases of ligand electronwithdrawing properties andor the steric demands near the cu center increase the e0 of the cu catalysts but decrease the rate of o2 reduction the kinetic rate of o2 reduction at e0 reported as kinetic current divided by catalyst redox charge decreases as e0 increases from 16 s1 measured at e0 in airsaturated solutions for adsorbed cuphen to 04 s1 for cu29et2phen the maximum value of e for which catalytic activity can be attained is estimated to be 350 mv vs nhe near e0 the kinetic current deviates from that expected if o2 binding were the sole ratelimiting step this indicates that one or more of the electrochemical reduction steps are rate limiting at potentials near e0