Two models for the anodic oxidation of metals in concentrated solutions are presented. The first one is proposed for the passivation range and emphasizes the competitive processes of film formation and dissolution occurring at different sites of the electrode surface via selective adsorption of anions from the electrolyte. These processes result in a second-order dynamic behaviour of the metal/film/electrolyte system, which produces a peculiar shape of the impedance spectra circling the origin of the complex plane. The model is able to reproduce quantitatively the impedance spectra during the active/passive transition of pure copper, antimony, and Bi-Sb alloys in concentrated H₂SO₄ solutions. The second model pretends to describe film growth and dissolution in the passivity region. It is assumed that a barrier layer grows through high field-assisted migration of vacancies. Ionic surface charges at the metal/barrier film and barrier film/electrolyte interfaces either enhance or retard the vacancy transport during film growth. The model is able to reproduce the ac impedance response of passive Bi, Sb and Bi-Sb alloys in concentrated H₂SO₄ solutions.