It is established that the adhesion of thermal barrier coatings (TBCs) is dependent upon the characteristics of the thermally grown oxide (TGO) that forms between the TBC (zirconia) and the corrosion resistant bond coat. Work has been carried using laser induced fluorescence out to investigate the properties of the TGO (normally alumina) as a function of ageing treatments. The zirconia layer is transparent to the laser light used and thus the laser light can cause fluorescence in the TGO layer present on the bondcoat, so that the method is non-destructive. Fluorescence in the TGO is caused by trace impurities of Cr in the alumina layer. The position of the spectral peaks generated is stress dependent so that residual stress in the TGO can be determined; the method is also known as piezospectroscopy. Residual stress maps were generated for an electron beam physical vapour deposited (EB-PVD) TBC that showed a large variation in residual stress over the surface of a coated sample. The two peaks generally associated with alpha alumina (RI and R2) frequently appear as doublets with a high and low stress component. In addition, the presence of a metastable theta-alumina was detected in aged samples. It is believed that these observations can be related to incipient spallation of the TBC. The development of residual stress and the metastable oxide have been studied and correlated with the spallation behaviour of the TBC. Additionally, the intensity of the spectra has been shown to yield information about the Cr content of the TGO, and this in turn could also be used as an indication of the remaining life of the TBC system.