To understand ''fatigue'' or ''burn-in'' in the target of a PbO Vidicon, experiments have been made with sandwich cells of evaporated PbO. It has been demonstrated by using chopped radiation that as the PbO layer fatigues, there is an increase in short term dark conductivity. Further, it has been shown that the phenomenon is associated with loss of oxygen from the surface of the crystals during irradiation with photons. A surface chemistry model similar to one proposed by Morrison and Melnick in connection with ZnO is used to explain the observed results. © 1962 The American Institute of Physics.