Aromatic nitriles, in particular phthalodinitriles, are prepared from an appropriately alkyl-substituted aromatic hydrocarbon by catalytic oxidation with oxygen or an oxygen-containing gas in the presence of ammonia at elevated temperatures in the vapor phase, in the presence of a catalyst which contains from 2 to 10% by weight of vanadium(V) oxide, from 1 to 10% by weight of antimony(III) oxide, from 0.02 to 2% by weight of an alkali metal oxide and alumina, by a process in which from 0.01 to 1.0% by weight of an alkaline earth metal, or of a compound of an alkaline earth metal, in particular barium or a barium compound, is added to the catalyst.