1. Heating to 525–950°C after austenitizing at 1050°C reduces substantially the stability of austenite. Destabilization of austenite is explained by a depletion of the solid solution in chromium and carbon by chromium carbide precipitation. 2. Prolonged holding at room temperature after austenitizing or heating at up to 500°C stabilizes the austenite. The martensitic transformation after thermal stabilization has the following peculiarities: Undercooling of the austenite down to −196°C is possible at relatively slow cooling rates; the transformation of austenite to martensite proceeds isothermally after a certain in induction period. The effect of isothermal temperature and holding time on the transformation rate is represented by a curve showing a maximum. 3. These rules do not apply to an ordinary martensitic transformation of unstabilized austenite. 4. Thus, in thermally stabilized steel Kh15N9Yu the martensitic transformation proceeds so as to indicate that its kinetics is determined by the thermal vibrations of the atoms.