The effect of high-temperature microstructure degradation (thermal ageing) on the corrosion resistance of austenitic stainless steels in supercritical water (SCW) was evaluated in this study. Mill-annealed (MA) and thermally treated (TT) samples of Type 316L and Type 310S stainless steel were exposed in 25 MPa SCW at 550°C with 8 ppm dissolved oxygen in a flowing autoclave testing loop. The thermal treatments applied to Type 316L (815°C for 1000 hr + water quench) and Type 310S (800°C for 1000 hr + air cool) were successful in precipitating the expected intermetallic phases in each alloy, both within the grains and on the grain boundaries. It was found that a prolonged time at relatively high temperature was sufficient to suppress significant compositional variation across the various intermetallic phase boundaries. This paper presents the results of the gravimetric analysis and oxide scale characterization using scanning electron microscopy (SEM) coupled with X-ray energy-dispersive spectroscopy (EDS). The role played by the fine precipitate structure on formation of the oxide scale, and thus corrosion resistance, is discussed. The combined role of dissolved oxygen and flow (revealed by examining the differences between Type 316L samples exposed in a static autoclave and in the flowing autoclave loop) is also addressed. It was concluded that formation of intermetallic phase precipitates during high-temperature exposure is not likely to have a major effect on the apparent corrosion resistance because of the discontinuous nature of the precipitation.