Impedance spectroscopy is a comprehensive technique that operates with frequency response (FR) to provide useful information about both surface treatments and layers produced. In this paper, two FR types were obtained for the aluminium samples during treatment by plasma electrolytic oxidation (PEO). The first was obtained in-situ during PEO, using a frequency sweep of bipolar voltage pulses between 20 Hz and 20 kHz. The modulus and phase angle components of the frequency response were analysed as functions of voltage and treatment time. With increased frequency, the impedance modulus and phase angle were found to decrease from 10(4) to 10 Omega and from 0 to -80 degrees respectively. The second FR was obtained using electrochemical impedance spectroscopy (EIS) of PEO coatings, at open-circuit potential. The samples were found to exhibit an active-capacitive load behaviour, whereby the impedance decreased from 10(6) to 10(2) Omega, and phase angle varied between -20 degrees and -80 degrees with the frequency increase. Both FRs show similar behaviour within the frequency range from 5 to 20 kHz. This approach allows estimations of the frequency response of the PEO microdischarges, to justify the optimal frequency for the pulsed bipolar PEO and also suggests concepts for feedback process control. (C) 2007 Elsevier B.V. All rights reserved.