The invention relates to a process for preparing phthalic anhydride, in which the catalytic gas-phase oxidation of o-xylene and/or naphthalene is carried out over at least three zones of catalysts of increasing activity, with only the last zone of the catalyst system containing phosphorus and the last zone also comprising at least 10% by weight of vanadium (calculated as V.sub.2O.sub.5) based on the active composition of the catalyst and having a ratio of vanadium (calculated as V.sub.2O.sub.5) to phosphorus of greater than 35.