The effects of compositionally induced changes on the semiconducting properties, optical response, chemical stability, and overall performance of KTaO3 photoanodes in photoelectrochemical (PEC) cells have been investigated. Single crystals of n-type Ca- and Ba-doped KTaO3 with carrier concentrations ranging from 0.45 to 11.5 x 10(19) cm(-3) were grown and characterized as photoanodes in basic aqueous electrolyte PEC cells. The PEC properties of the crystals, including the photocurrent, photovoltage, and flatband potential in contact with 8.5 M NaOH(aq) were relatively independent of whether Ca or Ba was used to produce the semiconducting form of KTaO3. All of the Ca- or Ba-doped KTaO3 single-crystal photoanodes were chemically stable in the electrolyte and, based on the open-circuit potential and the band-edge positions, were capable of unassisted photochemical H-2 and O-2 evolution from H2O. The minority-carrier diffusion lengths values were small and comparable to the depletion region width. Photoanodic currents were only observed for photoanode illumination with light above the bandgap (i. e., lambda < 340 nm). The maximum external quantum yield occurred at lambda = 255 nm (4.85 eV), and the depletion width plus the minority-carrier diffusion length ranged from 20 to 65 nm for the various KTaO3-based photoanode materials. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3089281] All rights reserved.