Two methods are proposed to determine the polarization resistances of electrochemical systems under linearization conditions from low frequency impedance measurements carried out under non-linear conditions, either using a polynomial fitting and extrapolation procedure to zero input amplitude or the odd harmonic criterion. Real systems are studied to test the theoretical formulations, for example a semi-conductor diode considered as a model system for exponential non-linearity and an electrochemical redox system with kinetic limitation by mass transport.