To investigate the initial degradation process in polymeric electroluminescent (EL) devices, in situ measurement of photoluminescence image change was carried out. The Al electrode was sunk or protruded during the device operation. Formation of the dented dark spot is observed by using an atomic force microscope and a scanning electron microscope, which is the critical factor of the degradation. (C) 1997 Elsevier Science S.A.