The growth kinetics of passive films on iron and stainless steel (Fe -16.31%Cr) substrates in pH 8.4 borate buffer solution was investigated by using in situ specular X-ray reflectivity. The oxide growth rate decays exponentially with increasing oxide thickness consistent with the point defect model in which the electric field in the oxide is maintained constant during growth. In stainless steel, however, the electric field depends strongly on the applied potential, indicating that the oxide properties change as the applied potential varies. Using the electric field in the oxide and the observed saturation oxide thickness in a quasi steady state, we estimate the potential drop at the metal/oxide interface, in the oxide, and at the oxide/solution interface.