The long term oxidation behaviour of TP 347H FG at ultra supercritical steam conditions was assessed by exposing the steel in test superheater loops in a Danish coal-fired power plant. The steamside oxide layer was investigated with scanning electron microscopy and energy dispersive X-ray diffraction in order to reveal the effect of oxidation time and temperature on the microstructure. A double layered oxide formed during steam oxidation. The morphology of the inner Cr-containing layer was influenced by the oxidation temperature. At temperatures below approx. 585oC, it consisted of regions of Fe-Ni-Cr spinel surrounded by Fe-Cr oxide. At higher temperatures almost the entire inner oxide layer was composed of Fe-Cr oxide. Possible mechanisms for the oxide growth are discussed and it is suggested that faster Cr transport within the alloy at higher temperatures explains the change in morphology. This hypothesis is supported by thermodynamic calculations and kinetic data. The thickness of the inner oxide layer did not change significantly with oxidation time and temperature for exposures less than 30000 h; however after 57554 h the thickness had increased significantly at the lowest temperatures.