A complete electrochemical oxidation-reduction cycle for a Cu monolayer on carbon-supported Pt clusters in alkaline solution has been monitored in situ by X-ray absorption spectroscopy. No desorption of Cu was observed upon oxidation. Near-range order and oxidation state of the Cu layer as a function of potential was obtained by in situ X-ray absorption spectroscopy. In contrast to bulk copper, the adsorbed copper monolayer transforms directly from the reduced state to Cu2+ and from the oxidized form to Cu-0. Adsorption of OH- ions on the Cu layer was observed for potentials below those for the Cu-0 --> Cu2+ and Cu2+ --> Cu-0 transitions.