Variations in the photovoltage and photocurrent of thin sputtered films of ZnS, sandwiched between sputtered Al electrodes and a thin transparent Au counterelectrode, have been studied as functions of the wavelength of the incident light. Reversal phenomena give evidence for the existence of two opposite photoeffects in these structures: an intrinsic effect across thc bandgap and an extrinsic effect depending on the nature of electrodes. Measurements were performed with a u.v.-photoeffect signal/noise ratio of more than 100 and a threshold discrepancy of less than 2%.