A process for preparing an organic target compound by heterogeneously catalyzed gas phase partial oxidation of an organic precursor compound with molecular oxygen in two oxidation reactor lines operated in parallel and removal of the target compound from the mixture of the product gas streams in a workup line, wherein the catalysts charge of one of the oxidation lines comprises a portion of catalyst over which the heterogeneously catalyzed gas phase partial oxidation has already been carried out for longer than over the portions of catalyst of the catalyst charge of the other oxidation reactor line.