A semiquantitative model is presented for the incorporation of species derived from electrolyte anions into amorphous barrier‐type films formed on aluminum in aqueous electrolytes at ambient temperature. The model relates the film compositions to the concentrations of adsorbed electrolyte anions which form the double‐layer charge at the film surface. During film growth, the adsorbed anions are incorporated into the film, either directly or following transformation to a new form, as so‐called electrolyte species. The incorporated electrolyte species present in the film can have positive, negative, or effectively no charge, and hence electrolyte species can be immobile, migrate inward, or migrate outward within the film under the electric field. The concentration of electrolyte species in the film depends upon the type and concentration of the adsorbed anions, the direction of migration of the electrolyte species in the film, and the faradaic efficiency of film growth. The validity of the model has been assessed by comparing the predicted and experimental compositions of films formed in a wide range of electrolytes. For reasonable selection of the type of adsorbed anion, the measured concentrations of electrolyte species in films, determined by Rutherford backscattering spectroscopy and nuclear reaction analysis, are typically about 70% of the predicted values, which is a satisfactory level of agreement given uncertainties in precise values of model parameters.