Cathodically reduced iron was anodically oxidized at potentials in the passive region of the anodic polarization curve in slightly basic sodium borate‐ boric acid solutions by means of a potentiostat. The kinetics of film growth were studied using an ellipsometer coupled with a fast recording technique, which enabled a determination of the rate of film formation for times less than 1 sec. This study of the kinetics of film growth using a nonelectrochemical technique agreed with kinetic results based on total charge measurements. Three stages of growth were detected: (i) First stage—growth limited by a diffusion process in the solution; (ii) Second stage—start of limitation of growth by the film involving the combination of several processes difficult to characterize by any rate law; (iii) Third stage—growth obeying either a logarithmic or inverse logarithmic rate law. The latter was found to imply that the passive film consisted of an outer layer poorly conductive for electrons.