Impedance spectroscopy measurements on free-standing plasma-sprayed thermal barrier coatings after annealing at 1250 degreesC for up to 1000 h have been performed to investigate the electrical properties in the temperature range from 200 to 350 degreesC. Microstructural properties have been characterized by SEM and XRD, bending strength and YOUNG's modulus have been obtained. The electrical impedance of the coatings is sensitive to changes in microstructure like densification, reduced microcrack density and phase transformations. Furthermore these effects can be separated from each other since they affect different elements in the used equivalent circuit model. Activation energies obtained from bulk and interlayer conductivities are affected by the annealing process. This method can be used to monitor changes in the top coat due to long term operation at high temperatures as well as in quality control applications. (C) 2004 Elsevier B.V. All rights reserved.