Raman marker bands characteristic of solid-state structure have been found for N, N'-di-1-naphthaleyl-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine(NPD), which is used as a hole-transport material in organic light-emitting diodes. The widths of the marker bands observed for an amorphous state at 1607, 1290, and 1192 cm(-1) are broader than those for the crystalline state observed at 1609, 1288, and 1198 cm(-1). These Raman bands are found to be useful for detecting the crystallization, which may cause degradation of organic light emitting diodes, of amorphous NPD films. (c) 2005 Elsevier B.V. All rights reserved.