The mechanism of phase transformations in cast heat-resistant alloy 45Х26Н33С2Б2 at the temperature of 1150°C is explained based on the results of experimental studies and calculated-and-theoretical analysis. It is shown that the factor determining the direction of the transformation of the structure (formation, growth and dissolution of phases) during long exposures to high temperature in alloys on the iron-chromium-nickel-base is the concentration of electron holes. The kinetically process is limited by the diffusion of the replacing item with the highest ratio of its concentration in a new phase and in the matrix (Сф/С0).