The influence of temperature and flow rate on the oxidation of 304L steel in O₂/H₂O mixtures was investigated. Polished samples were isothermally exposed to dry O₂ and O₂+40% H₂O at 500–800°C at 0.02–13 cm/sec flow velocity, for 168 hr. The samples were analyzed by gravimetry, XRD, ESEM/EDX, and AES depth profiling. The oxidation of 304L in water vapor/oxygen mixtures at 500–800°C is strongly influenced by chromium evaporation. The loss of chromium tends to convert the protective chromia-rich oxide initially formed into a poorly protective, iron-rich oxide. The rate of oxidation depends on flow rate; high flow rates result in an early breakdown of the protective oxide. The most rapid breakdown of the protective oxide occurs at the highest temperature (800°C) and the highest gas flow (4000 ml/min=13 cm/sec). The oxide formed close to grain boundaries in the metal is more protective, while other parts, grain surfaces suffer breakaway corrosion. The protective oxide consists of a Cr-rich 50–200-nm thick M₂O₃ film, while the parts experiencing breakaway corrosion form a 10–30-μm thick Fe-rich M₂O₃/M₃O₄ scale. The results show that chromium evaporation is a key process affecting the oxidation resistance of chromia formers and marginal chromia formers in O₂/H₂O mixtures.