The electrochemical behaviour of iron coated with a porous conversion layer has been studied in neutral aqueous solutions by means of Electrochemical Impedance Spectroscopy (EIS). The different structures of reactively formed layers of inorganic insoluble salts (ie phosphates) and of organic complex compounds (ie with phosphonates) can clearly be distinguished with this method. Using nonlinear least squares fitting all values of the components in the equivalent circuit model can be evaluated. In order to determine successively the components it is useful to combine the least squares fitting with a sequence of deconvolutions.