In situ scanning electrochemical microscopy (SECM) and scanning tunneling microscopy (STM) assisted scanning reference electrode technique (SRET) were applied for the first time to study the interaction of thiosulfate with Alloy 800 surfaces in aqueous chloride solutions. Electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM) were also performed to understand the interaction mechanism. The results showed that the effect of 0.075 mol/L thiosulfate in 0.6 mol/L chloride solutions strongly depended on the potential. There was no aggressive effect at the corrosion potential where the passive layer was intact, but a combined effect was observed with the presence of chloride ions at high potential where the passive layer was broken down. (C) 2015 Elsevier B.V. All rights reserved.