Microstructural evolution induced by isothermal exposure at 1100degreesC in a platinum aluminide coating was investigated. Specimens of CMSX-4 alloy have been platinum aluminised to give a single phase (Ni,Pt)Al coating. Coated alloys were exposed in an oxidising environment for 188, 350, 750 and 1500 hours at a temperature of 1100degreesC. After isothermal oxidation of the single phase (Ni,Pt)Al coatings the surface is shown to progressively roughen. The compositional development assisted by chemical diffusion during isothermal oxidation at 1100degreesC has been related to the transformation of the coating from its original beta-NiAl (132) structure to a Ni-rich (L1(0)) martensite gamma'. The transformation took place due to aluminium, depletion in the coating from the formation of the Al2O3 scale and interdiffusion between the coating and alloy substrate. The effects of phase changes on coating surface character are considered.