Using illumination at energies below the intrinsic diamond energy gap, photocurrent transients have been recorded for boron-doped diamond (BDD) as an electrode in an aqueous electrolyte of 0.1 M KH2PO4. The commercially-supplied BDD was in the form of a free-standing, polycrystalline film grown by chemical vapour deposition (CVD), with a boron acceptor concentration of >= 10(20) cm(-3). The effects of mechanical polishing of the BDD, of electrochemical hydrogen evolution and of electrochemical oxygen evolution (in 0.1 M KH2PO4), on the potential dependence of the photocurrent transients have been examined. Measurements of the cathodic photocurrent at light switch-on have been used to determine the photocurrent onset potential as a measure of the flatband potential. Comparison with and between related literature observations has shown broad agreement across considerably varying BDD/electrolyte systems. The flatband potential shifted positively following electrochemical oxygen evolution, indicating the formation of oxygen-containing groups on the diamond surface, these increasing the potential drop across the Helmholtz layer. For the electrochemically oxidised electrode, the cathodic photocurrent transient at a fixed potential changed reproducibly with changing solution pH, owing to the participation of the oxygen-containing surface groups in acid-base equilibrium with the solution. This clear demonstration of BDD as a photoelectrochemical pH sensor is in principle extendable to mapping the spatial variation in pH across a BDD surface by use of a focussed light spot. (C) 2013 Elsevier Ltd. All rights reserved.