Electrochemical impedance spectra were obtained for polyaniline membranes in weak and strong acid electrolytes. The measurement model approach for error analysis was used to show that the standard deviation of the real and imaginary parts of the impedance were equal, even for systems containing a large solution resistance with respect to the polarization impedance. These results were confirmed using analogue RC circuits that did not require use of the measurement model approach. A model for the error structure is proposed that is in agreement with data obtained for large and small values of the solution resistance.