The microstructure of the thermally grown oxide (TGO) of a FeCrAl-based thermal barrier coating model system (TBC-system) was studied by conventional and analytical transmission electron microscopy (TEM). The TGO scales of samples as deposited and after subsequent isothermal oxidation at 1 100 degreesC were analyzed regarding the scale microstructure. During oxidation, the TGO exhibited simultaneous inward and outward growth. Small metallic precipitates were found in the TGOs of both samples near the interface of an upper fine grained oxide layer (FGO) and a columnar grained oxide layer (CGO) below. Both observations are discussed with respect to phase transformations in the TGO and the oxygen partial pressure (pO(2)) gradient across the TGO scale.