Selective transformation of the methyl group into a formyl group in methylquinolines is of great interest because formation of the corresponding aldehydes is either very expensive or unknown. We report here results concerning the photo-oxidation of 6- and 8-methylquinolines (6-MQ and 8-MQ) suspended with TiO₂ powder in oxygenated acetonitrile. The analytical study of the products generated from the TiO₂-photoassisted oxidation of 6-MQ and 8-MQ in oxygenated acetonitrile suspensions indicated that in both cases, for a UV-illumination period of <24 h, the corresponding formyl derivatives quinoline-6- and -8-carbaldehydes are detected as practically the sole products. Analysis of further photogenerated oxidation products enabled mechanistic delineation of the course of the semiconductor-mediated reaction.