The effect of sulphur on the activity of fused iron catalyst in the reaction of the ammonia synthesis and decomposition has been studied. In both cases the non-linear dependence between the concentration of sulphur and the catalytic activity has been found. The deactivation has been very fast in the region of lower sulphur concentration and for higher concentration the quasi-stable level of the activity has been observed, depending on the temperature. The method of the determination of the number of active sites has been proposed, basing on the activity measurements on the poisoned and unpoisoned catalysts. The free enthalpy of the process of the active surface formation has been determined (21 kJ/mol for the ammonia synthesis and 63 kJ/mol for the decomposition). (C) 2002 Elsevier Science B.V. All rights reserved.