An analytical expression for the complex impedance of a uniform thin film containing two mobile charge carriers, as a function of the frequency. has been derived for the most general case. The film is surrounded by two media allowing interfacial exchange with both those species. Besides. the charging of the electrical double layers at the interfacial boundaries has been incorporated on the basis of the Gibbs thermodynamic equation. The impedance of a Film where such exchange with both species occurs only at one of the interfacial boundaries is obtained as a particular case of the general formula. These systems represent a model of mixed electron-ion conducting films (certain conducting or redox polymers. solid electrolytes, mixed valence oxides, lithium-ion intercalation or passivation films etc) in contact with one or two 'redox active solutions' (i.e. containing an electroactive Couple) so that both the Faradaic process and ion exchange take place at their interface(s). As another example one can point out solid ionic conductors containing two mobile cations (or anions) and a 'fixed' charge, both ions being exchangeable at the interface(s). The third example is provided by a thin membrane layer containing two mobile ion species which may be exchanged with the solution(s) in contact. The expression for impedance in the cases where only one of the mobile charge carriers can cross the interface, or no species at all can be exchanged ('blocking interface'), is immediately obtained from the general formula at the infinitely large value of the corresponding charge-transfer resistance.