An experimental reinvestigation (UPS and XPS) of the chemisorption of molecular oxygen on copper is described. Ultra-violet (HeI and HeII) photoelectron spectra of the oxidised surfaces are reported: XPS intensity data were used to monitor the uptake of oxygen, enabling an apparent discrepancy between previously published studies to be resolved. The spectra suggest that surface reconstruction occurs at ordinary temperatures even for very small oxygen uptakes. Semi-empirical calculations have been performed for bulk Cu and Cu2O, and for the (111) face of copper before and after the addition of an O monolayer. The valence electron band structures obtained are compared with experiment. The infinite crystal extended Hückel calculation for Cu metal reproduces the d-band well but provides a poor description of the s-band, suggesting that an enlargement of the basis set, in this and similar metal cluster calculations, would prove advantageous. For the copper + oxygen systems the effects of charge transfer, including the presence of a charged crystal lattice, were taken into account and a self consistent charge distribution was obtained for bulk cuprous oxide.