Both potentiostatically and galvanostatically controlled electrochemical impedance spectroscopy were used to show that the impedance obtained from passivated titanium electrodes is dominated by the capacitance of the space-charge layer. As a result neither the thickness of the oxide layer nor its rate of thinning can be readily determined from impedance measurements. Furthermore, it appears that the corrosion rates of passive metals determined by EIS may significantly overestimate the true rate of dissolution. No significant differences were observed between the impedance data obtained from the galvanostatic acid the potentiostatic measurements. (C) 2000 Elsevier Science Ltd. All rights reserved.