A catalyst for the preparation of phthalic anhydride by vapor phase catalytic oxidation of either ortho-xylene or naphthalene with a molecular oxygen-containing gas which comprises a catalytically active substance supported on a porous inert carrier, said catalytically active substance comprising 1 to 20 parts by weight of V2O5 and, correspondingly, 99 to 80 parts by weight of TiO2 and, based on the total weight of the V2O5 and TiO2, 0.02 to 5.0 % by weight of Nb2O5, 0.1 to 5.0 % by weight of P2O5, 0 to 0.25 % by weight of K2O and 0 to 0.5% by weight of Cs2O, the total weight of K2O and Cs2O being at least 0.05 % by weight and optionally 0 to 5.0 % by weight of at least one member selected from the group consisting of rare earth elements, ZnO and Tl2O.