Abstract: The interaction between Michler's ketone and chlorine-containing polymers has been studied by means of UV-VIS spectroscopy. No Michler's ketone-polymer charge transfer complex in the ground state was found.There appeared only a red shift of Michler's ketone in region of 365 nm. The degree of the red shift depends on the content of chlorine in the polymer. Upon exposure of the film consisting of Michler's ketone and polymers with various Cl content to a high-pressare Hg lamp a decrease of the～365nm peak bas been observed.The rate of decrease is directly proportional to the weight percent of the chlorine in the polymer. The same results have been obtained when the film is fumed with HCl gas. The absorption peak returned to a certain extent when the exposed films were kept at 50℃. The spectral response curve was found in a good coincidence with the Michler's ketone absorption spectrum. For spectral sensitization of Michler's ketone in the acidsensitive film the first step considered is the Michler's ketone in an excited state formed after the ketone has absorbed light energy.