The present paper considers the influence of irradiation by high-energy electrons on atomic distribution that is, short range order, of fcc Fe-Mn alloys. Using Mössbauer spectroscopy to measure the effective magnetic field distribution function, it has been shown that alloys with manganese concentration up to 45 atomic % electron irradiation decompose after leading to the formation of concentration inhomogeneities with regions depleted and enriched by manganese atoms. The atomic ordering process probably takes place in alloys almost equiatomic in composition.