The ammoxidation of ethane and ethylene to acetonitrile was studied over a Co-ZSM-5 catalyst with an emphasis on the reaction pathway. We found that the adsorption of ammonia on Co-ZSM-5 is stronger than that on H-ZSM-5. C2H4 adsorption is weak and readily desorbs below 300 degrees C. While the adsorption of C2H3N (a reaction product) is very strong in He, its desorption is accelerated with the presence of NH3. With a specially designed temperature programmed experiment (reaction between the adsorbed NH3 and gaseous C2H4/O-2/He mixture), we observed C2H5NH2 as a reactive intermediate, and this intermediate was demonstrated to be readily converted to C2H3N under the ammoxidation reaction conditions. A detailed pathway is offered, whereby C2H4 is thought to add on an adsorbed NH3, forming an adsorbed ethylamine which is subsequently dehydrogenated to form C2H3N. We further speculate that an oxidative environment N-2 comes from N-N pairing between the adsorbed NH3 and an amine (both on a single Co2+ site).