Isotopic transient experiments in vacuum of the ammoxidation of toluene were carried out, applying the temporal-analysis-of-products reactor (TAP) using alpha-((NH4)-N-14)(2)[(VO)(3)(P2O7)(2)] as catalyst and (NH3)-N-15-containing ammoxidation feed gas. The study revealed the participation of ammonium ions in the N-insertion step during benzonitrile formation. The benzonitrile formed during the ammoxidation pulses is composed of the N-14- and N-15-product, The share of the N-15-benzonitrile increased with pulse number.