The mechanism of the oxygen evolution reaction (OER) by catalysts prepared by electrodepositions from Co2+ solutions in phosphate electrolytes (Co-Pi) was studied at neutral pH by electrokinetic and O-18 isotope experiments. Low-potential electrodepositions enabled the controlled preparation of ultrathin Co-Pi catalyst films (<100 nm) that could be studied kinetically in the absence of mass transport and charge transport limitations to the OER. The Co-Pi catalysts exhibit a Tafel slope approximately equal to 2.3 x RT/F for the production of oxygen from water in neutral solutions. The electrochemical rate law exhibits an inverse first order dependence on proton activity and a zeroth order dependence on phosphate for [Pi] >= 0.03 M. In the absence of phosphate buffer, the Tafel slope is increased similar to 3-fold and the overall activity is greatly diminished. Together, these electrokinetic studies suggest a mechanism involving a rapid, one electron, one proton equilibrium between Co-III-OH and Co-IV-O in which a phosphate species is the proton acceptor, followed by a chemical turnover-limiting process involving oxygen-oxygen bond coupling.