The degradation of organic light-emitting devices (OLEDs) often correlates to the formation of dark spots in active area of the devices. An optical image analysis technique is developed to investigate the evolution of the dark spots. The circular features observed in the optical images of non-operated devices correlated directly to the dark spots seen in the same locations in the electroluminescent images. It is found that the growth of dark spots is dependent on the contact between the anode and the first organic layer. An ultra-thin interlayer of tris-(8-hydroxyquinoline) aluminum modified anode favors the efficient operation of the OLEDs. (C) 2004 Elsevier B.V. All rights reserved.