The passivation behavior of nickel in sodium sulphate solutions up to 290°C has been studied in an autoclave. An external reference electrode system was applied to the electrochemical measurements in the autoclave. The composition of the passive film formed on nickel was identified by the transmission electron diffraction analysis. The corrosion potential of nickel in the deaerated 0.1N Na₂SO₄ solution above 200°C was very close to the hydrogen reversible potential, indicating that nickel behaved as a hydrogen electrode. Under the presence of dissolved oxygen, the corrosion potential of nickel increased to higher potentials and the specimen showed a locallized attack. Anodic polarization experiments showed that the stability of the anodic passivity of nickel increased with increasing temperature. However, the typical pitting which will be caused by the aggressive nature of sulphate ions, was observed at relatively less noble potentials at 200°C. At temperatures above 250°C pitting occurred in the transpassive region. However, no indication of pitting was found in borate, phosphate, bicarbonate and nitrate solutions at 200°C. Transmission electron diffraction analysis showed that the passive film formed on nickel mainly consisted of NiO and unknown substances which are assumed to be hydroxides. With increasing temperature and anodic potentials, the number of diffraction rings decreased and those of NiO became predominant.