Thermal barrier coatings (TBCs) composed of an yttria partially-stabilised zirconia (YPSZ) top coat and either a beta -phase or a gamma-gamma'-phase Pt aluminide bond coat have been applied to a single-crystal superalloy (CMSX-4). Thermal cyclic oxidation tests are being carried out at 1200 degreesC in dry flowing air. From the results to date, the ceramic top coats have been found to be prone to spallation from both types of bond coat, with no significant difference in terms of time to failure between the two systems. Spallation in the TBC systems occurs mainly at the bond coat/ thermally grown oxide (TGO) scale interface but, occasionally, within the TGO scale or at the TGO/ceramic top coat interface. The various interfaces were carefully examined to determine any segregation of substrate elements, such as Re, Ti, Ta or W, and to understand better the causes and mechanisms of failure. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) have been used to acquire quantitative analyses and profiles across these interfaces.