The oxidation behaviour of Alloy 800 at 900 and at 980°C and the creep-rupture properties at 850°C were studied in air and in steam-charged oxygen atmospheres. Oxidation kinetics were determined by thermogravimetry. Creep-rupture-tested samples were investigated by scanning electron microscopy. The formation of oxide in creep-deformation areas was investigated by energy dispersive X-ray analysis. The oxidation rate of samples tested in air was governed by a chromia interlayer. Depending upon the partial pressure of the steam, the oxidation rate was dependent on H 2O partial pressure. Under these conditions a silica interlayer formed below the chromia interlayer. Viscous flow of the silica layer during creep testing may induce crack blunting in segmentating surface oxides. This mechanism may account for higher creep-rupture elongation in an atmosphere containing water vapour.