Kinetic curves of isothermal decomposition of undercooled austenife in chromium-nickel-molybdenum steels were obtained in a temperature range of 580 to 710°C. The effect of chromium and nickel on the parameters of the Avrami equation was studied. Based on a generalization of experimental data, isothermal diagrams were calculated, which enable prediction of changes in the stability of undercooled austenite within the field of diffusion decomposition upon various alloying of medium-carbon steels with chromium and nickel.