Deuterium labeling experiments indicate that two parallel pathways occur in the ammoxidation of propane over an alumina supported vanadium antimony oxide based catalyst, one in which the rate determining step involves hydrogen abstraction from the secondary carbon and the other in which it involves hydrogen abstraction from a primary carbon. Experiments show that d(0)-propane reacts faster than both 2,2-d(2)-propane (observed k(H)/k(D) = 1.7) and 1,1,1,3,3,3-d(6)-propane (observed k(H)/k(D) = 1.1), indicating that abstraction from the secondary carbon occurs 12-15 times more rapidly than the abstraction from a primary carbon. Scrambling of hydrogen and deuterium in acrylonitrile precluded the use of products in the rate analysis; the percentage of deuterated propane in reacted vs unreacted gas mixtures was used to determine the relative rates of reaction between the propanes used, Two different reaction feed mixtures, one with excess ammonia and oxygen compared to propane and one with excess propane compared to ammonia and oxygen, were used in order to ensure that high selectivities to propylene plus acrylonitrile were achieved and that the most selective pathway, not one leading to waste production, was being observed.