The research focused on changes in the structure, phase and chemical composition, activity and overheating resistance of the fused iron catalyst for ammonia synthesis in the ingot of the melted catalyst. The ingot is characterized with high heterogeneity. The distribution of promoters in the ingot is determined by the progress of crystallization. Aluminium is evenly distributed over the whole volume of the ingot. Other promoters shift along with the crystallization face. Their highest concentration is to be found in the middle part of the ingot. The content of promoters in the magnetite phase depends on the cooling rate of various parts of the ingot. When the temperature of cooling decreases, the concentration of aluminium in the magnetite also decreases while the concentration of calcium increases. Catalysts which are more slowly cooled, with a smaller amount of aluminium and a larger amount of calcium in the magnetite are less resistant to overheating. Catalysts from the upper and lower parts of the ingot, with the highest content of the magnetite phase, are more active than catalysts from the middle part with a significantly smaller portion of the magnetite phase. (C) 2011 Elsevier B.V. All rights reserved.