Flatband potentials of boron-doped polycrystalline and single crystal chemical vapor deposition and high-temperature, high-pressure diamond electrodes were estimated by three photoelectrochemical methods (i) as the photocurrent onset potential, (ii) from the dependence of photopotential on the light intensity, and (iii) by extrapolating the potential dependence of the photocurrent squared. The potentialities of these methods are discussed, as applied to diamond electrodes.