We investigate the origin of the surface conductivity of H-terminated diamond films immersed in aqueous electrolyte. We demonstrate that in contrast to the in air situation, charge transfer across the diamond interface does not govern the surface conductivity in aqueous electrolyte when a gate electrode controls the diamond/electrolyte interfacial potential. Instead, this almost ideally polarizable interface allows the capacitive charging of the surface. This description resolves the observed disagreement of the pH sensitivity of the diamond surface conductivity in air and in aqueous electrolyte.