Time-varying phenomena, such as crystallographic pitting corrosion, can be investigated by electrochemical impedance spectroscopy (EIS). For this purpose, a stepped sine excitation is most frequently used. However, when the frequency ranges from 25 Hz down to 15 mHz this experiment takes at least 30 min. In this work a periodic Schroeder multisine excitation signal is presented, by which the measurement time can be reduced to 2 min for the same frequency range. To evaluate the correctness of the EIS results obtained with the multisine, data was acquired for aluminum (99.5 wt.%) immersed in 0.5 M NaCl with oxygen bubbling by means of a stepped sine, next a multsine and finally a stepped sine excitation. From the spectra obtained with both stepped sines a new virtual spectrum is calculated as if all excited frequencies would have been recorded at the moment when the multisine was imposed to the system. A comparison of the multisine spectrum with this interpolated spectrum was then used for validation. The results show that the instantaneous behaviour of a time-varying system, observed during EIS is better approximated by the use of a multisine excitation signal. Moreover, the approximation further ameliorates when the immersion time evolves. (C) 2004 Elsevier Ltd. All rights reserved.