Isothermal bainitic transformation is studied in commercial and experimental alloy steels of various compositions using dilatometry. The effect of the conditions of cooling to the temperature of isothermal quenching on the subsequent kinetics of the bainitic transformation is considered. The experimental kinetics of formation of bainite is described with the help of the Austin-Rickett (AR) equation. The temperature dependence of parameters n and ln(k) in the AR equation and their linear interrelationship are presented. It is shown that the dependencies n = f[ln(k)] differ for steels of specific classes, and the difference may be associated with different stabilities of the supercooled austenite. It is shown that in a number of cases the kinetics of the isothermal bainitic transformation is describable adequately by the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation. The suggested differential equation generalizing the AR and KJMA equations makes it possible to describe the kinetics of formation of bainite the most accurately.