Luminescence measurements on 2-isopropylthioxanthone (ITX) in a poly(methyl methacrylate) (PMMA) film as a model system were carried out to study the secondary reactions of ITX during and after photo-crosslinking of organic coatings. The investigations have established evidence for two important secondary reactions in the photocuring of organic coatings using thioxanthone/amine as the initiating system: photoreduction of ITX by hydrogen abstraction from the polymer, and photodecomposition of peroxides and hydroperoxides sensitized by thioxanthone. -- AATA