A measurement model based on a broadband periodic excitation and applicable to electrochemical impedance spectroscopy was implemented. The model was able to detect, quantify, and separate the present measurement noise and stochastic non-linear contributions. By means of it, the impedance response of copper in neutral chloride solutions containing millimole additions of imidazole and pyrazole was measured. The results obtained showed that the different electronic structures and molecular conformations of the structural isomers studied significantly affected both the impedance spectra and the magnitude of the non-linear distortions. (C) 2004 Elsevier B.V. All rights reserved.