In this article, we present a simple method for the determination of the absolute internal quantum efficiency of thin organic dye layers. The basic idea is the comparison of the luminescence of the film with the reflection of a white diffusive reflectance standard measured at one angle with a simple spectrofluorometer. The method is compared to the procedure of de Mello [J. C. de Mello, H. F. Wittmann and R. H. Friend, Adv. Mater. 9, 230 (1997)], which uses an integrating sphere. As examples, the quantum yields of films of the two perylene derivatives N,N'-dimethylperylene-3,4:9,10-dicarboximide and 3,4,9,10-perylenetetracarboxylic dianhydride, are determined. (C) 2000 American Institute of Physics. [S0021-8979(00)08311-0].