We demonstrate in a photoemission electron microscopy (PEEM) study of Pt/YSZ in vacuum that an open circuit (OC) potential can develop which we attribute to an inner photoelectric effect caused by the irradiation with UV light The appearance of an OC potential is linked to the existence of a high voltage (HV) of at least 5 kV between sample and PEEM instrument The OC potential grows with increasing HV reaching about -200 mV at a HV of 12 kV. The effect represents a potential artefact in PEEM studies of solid ionic conductors with sufficiently small band gap (<6 eV for photons from D-2 discharge lamp). (C) 2016 Published by Elsevier B.V.