The conversion of isomeric chlorotoluenes to the corresponding benzonitriles by heterogeneous catalytic ammoxidation was studied. The reaction was carried out over (NH4)(2)[(VO)(3)(P2O7)(2)] as catalyst generated under the applied ammoxidation conditions by a solid-state reaction of the precursor compound VOHPO4 . 0.5H(2)O. Activity and selectivity of the ammoxidation of chlorotoluenes are shown to depend on the position of the Cl-substituents causing a changed accessibility of the methyl group and a possible different electronic stabilization of the reaction intermediates.