Degraded thermal barrier coating samples cut from different after-service gas turbine components are examined by both electron microscopy and impedance spectroscopy. There is a relationship between the microstructural and compositional features of the thermally grown oxide (TGO) and its electrical properties. The resistance of the TGO decreases with the TGO evolution from alumina to porous mixed oxides composed probably of NiO, spinel, Cr2O3, and Al2O3, while the relaxation frequency corresponding to the TGO increases. For seriously degraded TBCs, there is an additional semicircle in the impedance spectra in the extremely low frequency range, possibly arising from cracking in the vicinity of YSZ-TGO interface regions. (C) 2003 Kluwer Academic Publishers.