Impedance measurements of alumina scales on Fecralloy substrates were made at temperature from 400 C to 650 C. The alumina scales were also examined using scanning electron microscopy, transmission electron microscopy, coupled with energy dispersive X-ray analysis. Impedance spectra of the alumina scale in modulus format indicate that the thickness of the alumina can be measured based on impedance measurements whereas the change of impurity level in the alumina can be monitored according to variation of activation energy for electrical conduction. (C) 2008 Elsevier B.V. All rights reserved.