A novel expression of carbon diffusivity in pure ferrite has been derived with parameters fitted to known empirical equations and additional experimental data obtained by the tracer atoms method. The expression is then refined by allowance for the alloying effect on the activation energy revealed with available experimental data on Fe-X binary alloys (X = Cr, Mo, Ni, Si) and our investigation of strain aging kinetics in low-carbon automotive BH.steels with various contents of Mn and P.