Electrochemical impedance spectroscopy and microstructural evolution of thermal barrier coatings have been studied. The results of impedance spectroscopy measurement were fit to an idealized model circuit, which represents the physical behavior of thermal barrier coatings. Impedance spectroscopy was sensitive to defects such as cracks within yttria-stabilized zirconia coatings, and growth of thermally grown oxide film between the top coating and the bond coating. The experimental results provide a basis for the application of impedance spectroscopy in non-destructive evaluation of thermal barrier coatings. (C) 2003 Elsevier B.V. All rights reserved.