Semiconductive properties of the passive films formed on Alloy 800 in 0.1 M chloride solutions containing various concentrations of thiosulfate are investigated using Mott-Schottky analysis. The results indicate that when the concentration of the thiosulfate ions increases, the semiconductivity is converted from the n-type to the p-type. This conversion is postulated to be due to the incorporation of sulfur into the passive film, which changes the vacancy type. The breakdown behavior of the passive films is correlated with the semiconductivity of the film in this system. (C) The Author(s) 2015. Published by ECS.