A catalyst for the preparation of maleic anhydride by vapor phase catalytic oxidation of benzene with a molecular oxygen containing gas comprising a porous inert support of an alkali metal content, calculated as oxide, of at most 0.3% by weight and supported thereon a catalytic substance of a constituent composition comprising (a) 1 mole of vanadium pentoxide, (b) 0.3 to 1.2 moles of molybdenum trioxide, (c) 0.005 to 0.05 moles of phosphorus pentoxide, (d) 0.03 to 0.2 moles of sodium oxide and (d) 0 to 0.05 moles of potassium oxide.