Magnetic epitaxy films of Fe 4 N have been grown by molecular beam epitaxial in a flow of atomic nitrogen from a radio-frequency plasma source. Various mixtures of nitrogen and hydrogen have been used in the source during evaporation of 5 7 Fe on a MgO(100) substrate. The admixture of hydrogen leads to an increase of the uptake of nitrogen by iron and it improves the crystalline properties of the films grown. It was found that the increased nitrogen uptake is due to a higher fraction of atomic N in the output of the source plus transport system. The sticking coefficient of N atoms to an iron and a Fe 4 N surface was found to be equal to unity under the conditions we used. The composition and the morphology of the films was studied with conversion electron Mossbauer spectroscopy, X-ray diffraction, RBS/channeling and atomic force microscopy measurements. The results obtained clarify in part the growth mechanism of the iron nitride layers and the influence of hydrogen.