The valency of copper in freshly prepared as well as in situ high-temperature oxygen-treated 123 oxides was analysed by X-ray photoelectron spectroscopy. The results suggest the presence of Cu2+ and Cu3+ along with Cu+. This observation is supported by cyclic voltammograms of 123 oxides recorded in a formamide medium. The valence band of 1 23 oxides was probed using X-ray (Mg Kα) and ultraviolet (He-I) sources. It was observed that the Cu-O hybridized orbital in YBa2Cu3O7-x responsible for conduction decreases with time in an ultrahigh vacuum and increases with oxygen treatment temperature. © 1992 Chapman & Hall.