The iron catalysts for ammonia synthesis doped with manganese in amounts up to 3 wt.% has been investigated. The addition of manganese oxide did not influence the distribution of other promoters in the catalyst precursors, manganese oxide itself was entirely incorporated into phases of iron oxides, its concentration in the wustite phase was about twice that in the magnetite phase. At concentrations of manganese below 0.4 wt.% the activity of manganese-doped catalyst was about 10% higher than that of standard industrial catalyst in the temperatures 500 and 450degreesC after reduction, and in 400degreesC after overheating. At higher concentrations the manganese acted as a catalyst's poison. The cause for the higher activity of catalysts of low manganese content is surface area development, poisoning effect is due to occupation of active sites by manganese atoms at manganese content exceeding 0.4 wt.%. The corresponding surface coverage calculated according to Langmuir-Maclean's model of segregation amounted to theta = 0.25. The estimated value of free energy of segregation of manganese in iron in the overheated catalysts amounted to approximately - 15 kJ/mol. Before overheating no segregation process was observed. The surface coverage exceeding theta approximate to 0.3 caused formation of a new phase of manganese oxide on the surface of catalyst. Little effect of manganese addition on specific surface area and mean iron crystallite size is observed after reduction. After overheating manganese caused the rise of specific surface area due to higher energy of manganese-oxygen bond than that of iron-oxygen bond. Together with increasing surface area the mean diameter of iron crystallites also increased. (C) 2004 Elsevier B.V. All rights reserved.