The reduction of oxygen on copper in neutral unbuffered 1 mol dm(-3) NaCl has been studied using rotating ring-disc electrodes at six oxygen concentrations equivalent to atmospheres of 2% O-2 + N-2 to 100% 0(2). Steady-state potentiostatic measurements show that the reaction is first order with respect to [O-2] and that, following adsorption of O-2, the first electron transfer is rate determining. In 50% O-2 + N-2 and 100% O-2, a cathodic oxygen reduction peak is observed in both potentiodynamic and potentiostatic experiments at a disc potential of -0.3 to -0.4 V/SCE. The reaction is dominated by the overall four-electron reduction to OH-, with only small amounts of peroxide detected by the ring electrode at disc potentials corresponding to the formation of the cathodic oxygen reduction peak. Tafel slopes increase with [O-2] and vary from -0.13(5) V in 2% O-2 + N-2 to a limiting value of -0.16 V to -0.18 V in air, 50% O-2 + N-2 and 100% O-2. The results are explained by a mechanism involving oxygen reduction on two types of surface site with different reactivities. The most catalytic surface is believed to comprise Cu(0) and Cu(I) sites, where the Cu(I) species is stabilized as Cu(OH)(ads) and/or submonolayer Cu2O The less catalytic site consists of Cu(0) only. Oxygen reduction is believed to proceed by a series pathway involving an adsorbed peroxide intermediate on both sites. Peroxide is reduced to OH- prior to desorption at Cu(0) sites, but some is released before being reduced at Cu(0)/Cu(I) sites. Surface coverage by catalytic Cu(0)/Cu(I) species is favoured by a higher interfacial pH and more positive disc potentials.