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Thermal barrier coating (TBC) is applied on a surface of a gas turbine blade to provide a thermal barrier and oxidation resistant properties for the components. The ability to resist oxidation of the coating arises from the self-healing, protective Al2O3 scale on top of the bondcoat, which is formed during service. However, if Al depletion occurs within the coating, the protective scale will lose its self-healing ability, and hence, its oxidation resistant property. This paper investigated the depletion of Al within the bondcoat by studying the microstructure of the bondcoat on a gas turbine blade after it has been in 4,000 hour service at 1,200°C. The results showed that Al depletion had occurred at a different level throughout the turbine blade. In the area where Al depletion had not yet occurred, precipitation of a (Ni,Al)-rich phase was detected. Most of the Al was contained within this phase, leaving only small amount of Al in the surrounding matrix. A well-defined boundary was observed between the depleted and non-depleted regions.