A platinum-aluminide environmental coating has been subjected to simulated operating thermal cycles normally encountered in land-based gas turbines. The oxidation was undertaken in a mixture of gas with 6 vol.% steam. The specimens were subject to long term cycles (1000hrs) at temperatures 850°C, 900°C, 950°C and 1050°C for up to ten cycles. Growth of the thermally grown oxides with thermal exposure is explored, and the fracture and delamination modes of the protective oxide layer were characterised by scanning electron imaging combined with focus ion beam milling. Two representative failure modes of the thermally grown oxide are proposed and discussed.