In situ Mossbauer spectroscopy has been used to examine the active phase of two different reduced 4% Fe/SiO2 catalysts for acetonitrile synthesis from CO, H-2, and NH3. The Fe-0-containing, silica-supported catalyst starts with a finite initial activity and displays a maximum in activity with time that tracks the amount of the epsilon＇-Fe2.2C phase, indicating it as the active phase of the catalyst. The silica-supported catalyst containing Fe2+ only starts with a low activity and slowly rises to a stable activity but shows little change in the bulk structure of the catalyst. In this case, much of the change occurs on the surface of the catalyst and could not be monitored with Mossbauer spectroscopy. An increase in the reducibility of iron with time was confirmed, however, with H-2 desorption experiments. For reduced 11.9% Fe/carbon, also known to have epsilon＇-Fe2.2C as the active phase for acetonitrile synthesis, spectra collected at cryogenic temperatures after 3 and 10 min of reaction indicate the following order in carbide formation: Fe-0-->epsilon＇-->chi-->theta. Transient mass spectrometry used to monitor the product stream in the first 5 min of reaction indicates that carbide formation precedes product formation. Reactions carried out over rereduced, postreaction 11.9% Fe/carbon indicate large-scale sintering during the first cycle of reaction. Mossbauer studies also show a rapid rearrangement of the bulk structure of the rereduced used catalyst when it is reintroduced to the reaction environment.