The reaction pathways in the ammoxidation of propene and of propane over an Sb-V-oxide catalyst were studied by analyzing the modes of transient responses of reactants and products resulting from step changes from inert gas to reactant feed. The pathway from propane to acrylonitrile begins with the oxidative dehydrogenation of propane to form propene. After readsorption, the propene is then transformed into adsorbed acrolein, which eventually reacts with an NHx surface species to form acrylonitrile. The adsorption of propane is rate limiting for the propane conversion, but the desorption of water formed from the propene is slow and determines the product distribution. The response modes for the direct oxidation and ammoxidation of propene are consistent with the desorption of water being rate limiting. A comparison of the experimental responses viith responses simulated under the assumption that the formation of an allylic intermediate is rate determining reveals an important difference in profiles, indicating this step to not be rate determining. The results show that substantial qualitative information concerning the reaction pathways can be obtained from the analysis of transient profile modes.