The formation, growth, and dissolution of the passive film on iron in neutral and alkaline solutions was investigated using a light reflectance technique. In this study, laser light of 325 nm was used to access the amount of light absorbed in the film and to evaluate the optical absorption coefficient. After calibration, this technique allows a fast, in situ determination of the film thickness. Cyclic voltammetry and potential-step experiments with simultaneous thickness monitoring were carried out in berate buffer and in 0.1 M NaOH. For the berate buffer, anodic film formation and cathodic film dissolution are reversible, i.e., repeated anodic/cathodic cycling leads to conditions identical to the initial state. in NaOH, cathodic reduction of the film does not lead to film dissolution but to the formation of an electrochemically active layer, which does not significantly contribute to the potential drop in the metal/film/electrolyte system. Hence, repeated passivation and reduction leads to successive thickening of the film, consisting of a barrier layer of constant thickness and a porous layer having increased thickness with every passivation/reduction cycle. This layer does not contribute to the passivity of iron.