The reduction of oxygen on copper in neutral oxygenated 0.1 mol dm(-3) NaClO4 has been studied potentiodynamically with rotating ring-disc electrodes. Measurements of the interfacial pH during oxygen reduction have also been made with a Bi2O3 pH-sensing ring electrode. In unbuffered solution, the interfacial pH increases due to oxygen reduction to OH- and a catalytic surface film forms at potentials in the joint-control region at an interfacial pH similar to 10. The catalytic surface is believed to comprise Cu(OH)(ads) and/or submonolayer Cu2O. The formation of this catalytic surface results in an increase in the rate of oxygen reduction, and a cathodic peak is observed on the positive-going potential scan. On the negative-going potential scan oxygen reduction is inhibited, apparently on a complete Cu2O monolayer. In pH 7 phosphate-buffered solution, no change in interfacial pH occurs during oxygen reduction and no evidence for a catalytic surface film is observed. Consequently, no cathodic peak is formed during oxygen reduction in buffered solution.