Processes such as anodic dissolution, prepassive film formation (salt and/or oxide, hydoxide film), and passive film formation are in the initial stage controlled by diffusion. Information about the value of the limiting current density (i(d)) is needed for understanding the kinetics of passive film formation. In this study an alternative method, suitable for the determination of the i(d) through potential step measurements, is proposed. The method is based on the assumption that the reaction cannot proceed faster than that allowed by the i(d). In this case, the maximum reaction rate corresponds to the current density at the inflection point of the i-t curve, and this current density is identified as the limiting current density. This method was used to determine the i(d) values of sonic Cr-13% stainless steels at five passive potentials in mixtures of 0.1 M H2SO4 + 0.1 M Na2SO4 with different pH values. The results for the limiting current density were compared with the corresponding i(d) values obtained from the diffusion overvoltage equation. The agreement between the two methods is satisfactory.