The present invention relates to a catalyst system for the manufacture of carboxylic acids and/or carboxylic acid anhydrides, which has at least four catalyst layers which are arranged one on top of the other in the reaction tube, with the ratio of the packed bed lengths of the selective catalyst layers to the packed bed lengths of the active catalyst layers lying between 1.4 and 2. In addition, the present invention relates to a method for gas-phase oxidation, by which a gasiform flow, which contains hydrocarbon and molecular oxygen, is conducted through several catalyst layers, with the ratio of packed bed lengths of the selective catalyst layers to the packed bed lengths of the active catalyst layers lying between 1.4 and 2.