Anodic polarization curves of a carbon steel (JIS G 3106 SM 400B, 0.12 mass% C) have been measured in a boric-borate solutions (pH 8.45), dilute NaOH solutions (pH 8, 9, 10), water in contact with bentonite (pH 8.2-8.3), and mixed solutions of 5.72mol⋅m-3 Na2SO4 +7.99mol⋅m-3 NaHCO3 (pH 7, 8, 9, 10), which simulate the water in contact with bentonite. The formation and breakdown of passive films on the steel have been examined by in-situ ellipsometry. It was found that in the boric-borate solution of pH 8.45 the steel readily passivates with the formation of passive films. The thiclsness of the passive films increases almost linearly with increasing potential, and at the same time the real and imaginary parts of optical constants of the films tend to increase. In the water of pH 8.2-8.3, which had been in contact with bentonite for 10 and 60 days, the steel dissolved actively from the corrosion potential and did not passivate. Similar polarization characteristics were observed in 5.72mol⋅m-3 Na2SO4+7.99mol⋅m-3 NaHCO3 solutions of pH 7-9 and 5.72mol⋅m-3 Na2SO4 solution of pH 9, while the active-passive transition took place at 0.1 V in 7.99mol⋅m-3 NaHCO3 solution of pH 9. The anodic dissolution rate at potentials ranging from -0.4 to 0.1V in the Na2SO4+NaHCO3 solution of pH 9 was higher than that at correspon ding potentials in the NaHCO3 solution having the same pH value. These results suggest that SO42- ions promote active dissolution and impede the formation of passive films. Passivation occurred more easily when pH value was increased up to 10. In solutions containing SO42- ions, as in Na2SO4+NaHCO3 solutions, however, the steel suffered from pitting immediately after passivation was completed. The depassivation of the steel prepassivated in the boric-borate solution of pH 8.45 occurred by SO42- ions when the steel was immersed in aerated Na2SO4+NaHCO3 solutions of pH 7-10. No apparent change in the thickness and optical constants of the films was observed during the depassivation process, indicating that the depassivation occurs through local breakdown of the films.