The influence of sulfur on the activity of iron catalyst in the reactions of ammonia synthesis, ammonia decomposition and iron carburization in methane has been studied. The activity of iron catalyst in all reactions under study drops with increasing concentration of sulfur. The normalized activity of iron catalyst poisoned with sulfur depends on temperature: the higher the process temperature the higher normalized activity. The apparent activation energy of all considered processes increases with sulfur content in the catalyst. Simple site-blocking mechanism of poisoning cannot be applied, therefore, a complex mechanism of poisoning is proposed. It comprises a superposition of the site-blocking, the formation of sulfur-alkali metal compounds and a change of the number of active sites driven by temperature. (c) 2007 Elsevier B.V. All rights reserved.