Complex coating systems are required to protect nickel-based super alloys from high temperature oxidation and corrosion. Industrial gas turbine blades and heat shields are generally plasma sprayed with a metal bond coating containing nickel, chromium, cobalt, aluminium and yttrium, and then an external thermal barrier coating of yttria-stabilised zirconia is applied. In this study, samples of an IN939 alloy heat shield with both a metal bond coat and a ceramic thermal barrier coating have been heated in air at high temperature for up to 2000 hours to assess the long term stability of the metal bond coat. Polished sections of the heat treated samples were examined by SEM and EDX to determine microstructural changes. The Ni-Cr-Co-Al-Y coating was found to be a very effective barrier against oxidation; the only apparent oxidation being the growth of an alumina layer between the bond coat and ceramic thermal barrier coating. With time, the growth of the gamma＇ Ni3Al phase in the metallic bond coat was observed, with extensive diffusion of other elements to and from the bond coat.