Photodegradation and demethylation reactions of methylene blue (MB) by a TiO2 photocatalytic film have been studied spectrophotometrically by estimating molar concentration changes of MB and demethylated species in an MB aqueous solution according to the Beer–Lambert law. On the other hand, MB was less demethylated by the reaction with an Au particles-TiO2 (Au–TiO2) composite film.