Nickel-based alloys are being considered as construction materials for various components in high-efficiency steam turbines with envisaged operating temperatures around 700 A degrees C. In the present study, the steam oxidation behaviour of the nickel-based alloy 625 in the temperature range of 700-800 A degrees C was investigated whereby exposures up to 10000 h were carried out. Gravimetric data in combination with results from a variety of post exposure analysis techniques showed in all cases the formation of protective oxide scales mainly consisting of chromia with minor amounts of outer Cr/Mn spinel and internal silica. The phases found in the bulk alloy after long-term exposure were mainly needle-shaped delta-Ni-3(Nb,Mo) phase, mu-phase and Si-rich eta-M6C carbide. Microstructural features and phase formation were found to be related to minor variations in the alloy composition, especially iron and silicon content. The oxidation-induced chromium depletion caused a number of microstructural changes in the subsurface depletion layer. Most important was an enrichment of the intermetallic delta-phase at the scale-alloy interface. DICTRA modelling revealed this effect to be caused by uphill diffusion as a result of a negative niobium activity gradient in the subscale chromium depletion zone. Although the available kinetic and thermodynamic data allowed qualitative explanation of the delta-phase enrichment, the databases do not correctly describe the high molybdenum solubility in the delta-phase.