The kinetic study of the tempering reactions of a low-alloy steel (AISI 1050) was carried out through non-isothermal dilatometry. The kinetic parameters of the first and third stages on tempering (here referred to as processes I and II) are calculated by procedures which assume that the nucleation and growth reactions obey a Kolmogorov-Johnson-Mehl-Avrami (KJMA) kinetic model. The recipes to obtain the kinetic parameters (E, K (0), n) of the reactions on tempering are presented. The first stage of tempering is characterized by the growth of the transition carbide nuclei formed during the quenching, n = 1 (site saturation situation). This stage is controlled by the pipe diffusion of the iron atoms. The third stage of tempering is characterized by the cementite nucleation on dislocations due to the gradual dissolution of the transition carbide, n = 0.66. The cementite growth is controlled by diffusion of the iron atoms through dislocations and in the matrix.