The effects of electrochemical systems non-linearities on impedance measurements with a Frequency Response Analyser have been investigated in low frequency domain from both a theoretical and experimental viewpoint. A theoretical expression for the polarization resistance measurement error has been found for "non-blocking" electrodes and the measurement validity conditions have been examined. The theoretical approach has been applied to redox reactions and to an adsorbed phase reaction which is a dissolution-passivation process of a metal. The theoretical results were verified with impedance measurements on a platinum rotating disk electrode in a Fe(CN)63-/Fe(CN)64- redox solution.