Sum frequency generation (SFG) spectroscopy has been used to study the operational degradation of 4, 4 '-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (alpha-NPD)/tris(alpha-hydroxylquinoline) aluminum (Alq(3)) organic light-emitting diodes (OLEDs). A reduction in the SFG peak strengths of Alq(3) was found in aged OLEDs, indicating that the permanent dipole orientation in the Alq(3) layer was randomized as a result of long-term continuous operation. In contrast, the SFG signals derived from the alpha-NPD in aged samples were enhanced as compared to those of pristine OLEDs, and it can be attributed to an increase of the internal electric field near the interface by the long-term operation. (C) 2014 Elsevier B.V. All rights reserved.