In a previous investigation, three Fe-Cr alloys with various Cr contents were examined in 0.5 M H2SO4. These studies were based upon the analysis of ac impedance and frequency resolved rrde (Rotating Ring-Disc Electrode), determined simultaneously at different potentials in the active-to-passive transition range. The experiments provided quantitative information about the adsorption of reacting intermediates from the ac response of anodic current and electrical charge involved in the film formation. Based on the experimental results, namely the electrode impedance and emission efficiency, it was calculated that ferrous species were leaving the electrode surface with a current efficiency greater than unity at intermediate frequencies. Then, the differential capacitance, ie, the potential dependence of the electrical charge stored at the electrode surface as reaction intermediates, was evaluated. The model proposed in this paper describes the main features of the characteristic behaviour of the electrode process in a simplified manner rather than giving a detailed description of the full reaction. The simulated model is fitted to the experimental polarisation curve and thereafter to the impedance. Then the emission efficiency and the differential capacitance were calculated.