The kinetics of phase transformations is usually described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The article shows that this equation cannot give a sufficiently general description of austenitization kinetics of ferritic nodular cast iron. Therefore, another kinetics equation is proposed which catches the main circumstances and substance of austenitization more accurately than the JMAK equation does. It shows that the crucial phenomena in the transformation are not only the processes of the creation and growth of new austenite grains but also the change in specific volume and the chemical liquation of alloying additives, which retard the subsequent conversion. The proposed equation together with the Arrhenius equation allows describing simultaneously the temporal and temperature dependence of austenitization conversion including the partial transformation at insufficiently high overheating of transformed iron. It is verified by successful regression of experimental data, whose results allow drawing predictive curves for temperatures from the experimental temperature region or from its near vicinity, for which the conversion was not determined.