We studied the sensitization mechanism in a photo initiating system that consists of an aminostyryl sensitizing dye, 2-[p(di ethylamino)styryl]naphto[1,2-d]thiazole (NAS), and a radical generator, 2,2'-bis(2-chlorophenyl)-4,4',5,5-tetraphenyl-1, 1'-bi-1H-imidazole (HABI). In a photopolymer bearing NAS/HABI system, an improvement of the photosensitivity was observed by the addition of co-initiator, 2-mercaptobenzothiazole (MBT), to the initiating system. Photochemical and photophysical behavior of the three-component initiating system, NAS/HABI/MBT, in the PMMA film was studied by means of a nanosecond laser flash photolysis method employing a total reflection sapphire plate. It is considered that MBT can enhance to generate the triplet state of NAS, which is hardly observed by the direct excitation, and the mechanism of both singlet and triplet electron transfer from NAS to HABI is suggested. (c) 2005 Elsevier Ltd. All rights reserved.