A new model of the surface of a fused iron catalyst for ammonia synthesis is proposed. It is assumed that the iron surface is covered with a double layer due to wetting by promoter oxides. The first sublayer in the direct vicinity of the iron atoms is filled with oxygen atoms and the free adsorption sites are present in this sublayer. The second sublayer located over the first one is filled with promoter atoms. The number of oxygen atoms and free adsorption sites is determined by the nature of the promoter. Every atom of alkali metal needs one oxygen atom to be stable on the iron surface under ammonia synthesis conditions; one atom of alkali earth element needs two oxygen atoms; one atom of aluminum needs three. The number of the free adsorption sites is directly related to the number of oxygen atoms on the surface. The more oxygen atoms on the iron surface, the less free adsorption sites are available for dinitrogen adsorption. The surface area of the iron catalyst is also dependent on the number of oxygen atoms on the surface. Therefore, elements such as Ca and Al, which need a high concentration of oxygen on the surface, lead to the formation of the catalyst exhibiting the highest surface area. Between the surface, the iron bulk, and the spacers located between the iron crystallites the thermodynamic equilibrium is maintained. On the basis of this model an interpretation of some properties of the iron catalyst is presented.