Many catalysts are promoted by the addition of small amounts of alkali compounds which are known to change the selectivity of heterogeneous reactions in a favorable way. For example potassium is used as promoter for the iron catalyst in the Fischer Tropsch or ammonia synthesis. Taling into account an important radius of potassium atoms, they do not dissolve in the crystal lattice of iron and remain on the surface. Thus, the surface diffusion of potassium at higher temperatures is crucial for catalyst activation and deactivation. This paper is focused on the diffusion of potassium on the surface of ammonia synthesis iron catalyst studied under ultrahigh vacuum, under atmospheric pressure and under the pressure of 10 MPa. Under UHV the diffusion of potassium from iron catalyst to the iron foil has been studied using AES method. The AES spectra were taken from the iron surface in the temperature range 500-600 0C. The concentration profiles of potassium were observed in time. The diffusion of potassium on the iron surface was simulated using one dimensional second Fick's equation. The diffusion of potassium with a gas stream (nitrogen-hydrogen mixture) has not been observed at the temperature of 520 0C under atmospheric pressure. The intergranular diffusion of potassium has been proved during the ammonia synthesis reaction under the pressure of 10 MPa at the temperature above 400 0C.