Analysis of relative number of active sites of ammonia synthesis iron catalysts was performed by TPD-H(2) method. Samples of the industrial pre-reduced catalyst, which were reduced and heated at different temperatures from the range 500 degrees C to 780 degrees C in order to modify their specific surface area, were used. Chemical composition of samples to be tested was modified as well by adding sulfur compound or reducing the amount of potassium oxide. The process of hydrogen adsorption was conducted at 25 degrees C. Then hydrogen adsorption occurs on the surface together with absorption in the volume of iron. On the spectrum of H(2)-TPD the presence of peaks alpha(1), alpha(2) and alpha(3) was observed at temperatures 130 degrees C, 300 degrees C and 450 degrees C, respectively. Peak alpha(1), associated with adsorption on the surface, is proportional to the total amount of absorbed as well as adsorbed hydrogen. Height of alpha(1) peak varies with the shift of catalysts' activity. This peak gives the information about the number of active sites on the catalyst's surface. Heights of peaks alpha(2) and alpha(3) depend mainly on the size of sorption of hydrogen in the volume of iron. Changes in activity of the samples did not significantly affect the heights of peaks alpha(2) and alpha(3). (C) 2010 Elsevier B.V. All rights reserved.