[en] Using X-ray structure analysis, the type of crystalline lattice of α-martensite in Fe(1-14%)-Mn alloys was investigated with a view to determining the tendency of iron-manganese alloys to brittle failure. The dilatometry and the optical microscopy were also employed. It was found that the lattice of α-martensite in the considered system is tetragonal, this supporting the suggestion that the degree of tetragonality depends on the content and the character of distribution in the solid solution of substitutional elements, in particular, of Mn. Presented are the concentration dependences of the α-phase unit cell parameters