Photoelectrochemical characterization of semiconducting, diamond was carried out in a weakly UV-absorbing aqueous electrolyte using suprabandgap illumination in order to examine the influence of electrochemical oxidative surface treatment on the energetic positions of bandedges. Anodic treatment of diamond photoelectrodes resulted in a positive shift in both the photocurrent onset potential and the flatband potential obtained from Mott-Schottky plots, indicating the displacement of the bandedges. A corresponding increase in the photovoltage was also observed.. The pH dependence of the flatband potential of anodically treated diamond indicates an acid-base equilibrium at the interface, suggesting the formation of oxygen-containing groups. The presence of the latter was confirmed using X-ray photoelectron spectroscopy. These results suggest that the changes in the potential drop in the Helmholtz layer are due to oxygen functional groups, and the loss of surface and subsurface hydrogen are mainly responsible for the observed shifts in the flatband potentials.