Photoelectrochemical modification procedures at p-InP (I I I)A-surfaces for improved photocurrent-voltage behaviour in acidic vanadium(2+/3+) redox electrolyte are presented, Preconditioning in H2SO4 and HCl results in marked differences of the I-V characteristics in the redox-as well as in the supporting electrolytes: in H2SO4, the photocathodic reaction of hydrogen evolution is accompanied by photocorrosion leading to metallic indium at the surface. Cyclic polarisation in HCl, however, results in an improved photovoltage and fill factor. A new two-step conditioning procedure is presented yielding a solar-to-electrical conversion efficiency of 11.6% in a two-electrode non-optimised configuration. Electrochemical analyses of the dark current - voltage curves and Mott-Schottky plots show shifts of the flatband potential due to the surface conditioning. In HCl-cycled electrodes, an increased barrier height Phi(bh), is observed whereas upon cyclic polarisation in H2SO4. Phi(bh) decreases. In HCl, the formation of a passivating interfacial film of thickness of 5-7 Angstrom is assumed. the thickness estimate depending on the assumed corrosion reaction. The film allows efficient charge transfer and also improves the anodic protection of the p-InP electrodes in forward direction. The chloride ion appears to strongly influence the surface chemistry with respect to film formation.