Sparingly alloyed high-manganese austenitic steels 40G20 and 25G20S3 are studied. The phase composition, the structure and the mechanical properties of the steels are determined after quenching from the austenitic range, and their variation during tension of standard specimens is studied by the methods of transmission electron microscopy and x-ray diffraction analysis. The characteristics of strain hardening of the steels are determined from the results of mechanical tests. It is shown that the strain develops either by twinning (TWIP-effect, steel 40G20) or due to formation of strain martensite (TRIP-effect, steel 25G20S3) depending on the stacking fault energy. The change in the strain mechanism changes the strain hardening and the strength and ductility properties of the steels.