The beneficial effect of the addition by ion implantation of cerium on the corrosion behaviour of a 32Ni-20Cr austenitic steel in a mixed oxidizing/sulphidizing environment, at 700 "C has been studied. Emphasis was placed on the initial stages of the corrosion process using a wide range of surface analytical techniques. Comparison is made with unimplanted material and also with xenon-and chromium-implanted steel. The latter had no significant effect. In contrast Ce ion implantation improved the corrosion behaviour, which is probably due to a change in the nucleation stage of corrosion, leading to an improved quality of the oxide layer forming under the initial corrosion products. Factors which might contribute to this are discussed.