The formation and reduction of passive layers on copper in weakly alkaline solutions saturated with N-2 and O-2 were studied. Voltammetric and ellipsometric techniques were employed to examine the structural characteristics of the layers formed in the -0.32 to 0.75 V vs RHE potential region. Optical measurements at open circuit potentials (E(oc)) were also made to simulate operational conditions. The passive layer consists of a duplex structure : an outer hydrated copper oxide film and an inner dehydrated film. This inner layer is composed of Cu2O with a surface excess of Cu(II) ions. The growth rate of the oxide layers at controlled potentials is higher in O-2 saturated solution. The corrosion resistance of copper depends on the presence of O-2 in the electrolyte, on the stirring rate and on the E(oc) value.