We present the results of an x-ray diffraction investigation of the formation of oxide on electropolished Cu(111) surfaces, both in situ at pH 4.5 and in air. In both cases the oxide is found to be crystalline cuprite and epitaxially aligned with the substrate, but with two possible epitaxial orientations. We followed the progress of oxidation by monitoring the shapes of the diffraction peaks for the two orientations as a function of time and potential. There is a narrow potential region where the oxide is a single monolayer thick. Beyond that, only one of the two oxide orientations becomes thicker, and does so in an inhomogeneous manner, thickening in narrow regions before it spreads.