Though impedance is only defined for linear systems, impedance spectroscopy is also successfully applied to nonlinear systems such as fuel cells and batteries. The influence of nonlinearities on measurement results in impedance spectroscopy is therefore discussed on a theoretical and simulative basis. The basis is a simplified Randles model of an electrochemical cell, on which a simulated impedance spectroscopy in galvanostatic mode is performed. For the investigation the focus is on the Butler-Vollmer equation in order to describe the nonlinearity. Furthermore, a linear model for comparison is used, in which the Butler-Volmer nonlinearity is replaced by a linear resistor to show the differences in impedance measurement. In order to find a correlation, also the occurring harmonics are observed. The results are discussed and several methods are suggested for maintaining a quasi-linear impedance measurement by controlling the amplitude of the excitation signal. (c) 2008 Elsevier Ltd. All rights reserved.