An investigation was carried out to determine the failure mechanism of a thermal barrier coating system on an Ni-base superalloy. The coating system consisted of an outer layer of yttria-stabilized zirconia (top coat), and an inner layer of Pt-aluminide (bond coat). Specimens were exposed at 1010 and 1150 degrees C with a 24-h cycling period to room temperature. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used in microstructural characterization. Spallation of the oxide scale developed by the bond coat was found to be the mode of failure. Experimental results indicated that the breakdown of oxide was affected by internal oxidation of Hf diffusing from the alloy substrate into the bond coat surface developing localized high levels of stress concentration at the oxide-bond coat interface. It was concluded that the cause of failure was degradation of thermal stability of the bond coat accelerating its oxidation rate and permitting outward diffusional transport of elements from the sutstrate.