Abstract: 3-Methylquinoline-8-carboxylic acid I ##STR1## and derivatives thereof are prepared by reacting o-toluidine II or a derivative thereof ##STR2## with methacrolein II ##STR3## in 70-90% strength by weight sulfuric acid in the presence of iodine or of an iodine compound, and oxidizing the resulting 3,8-dimethylquinoline IV or a derivative thereof ##STR4## with nitric acid in a solution containing sulfuric acid, in the presence of vanadium ions, by a process in which, in the sulfuric acid-containing reaction solution of the 3,8-dimethylquinoline IV, the byproducts of the reaction are first decomposed by oxidation, the resulting 3,8-dimethylquinoline solution is concentrated by distillation and the 3,8-dimethylquinoline is then oxidized in situ with nitric acid to give the 3-methylquinoline-carboxylic acid.