In the present part of study, the effect of thiosulfate ion on pitting corrosion of AISI 316 stainless steel in 0.1 M NaCl solution was investigated. In order to elucidate the effect of thiosulfate on pitting corrosion, the metastable pitting has been studied in more details. The results showed that the thiosulfate addition increased susceptibility to pit initiation and increased the dissolution kinetics of metastable growing pits in a fashion that facilitates the transition from metastability to stability. Consequently, the pitting potential of the stainless steel was decreased in the presence of thiosulfate ion. (C) 2015 The Electrochemical Society. All rights reserved.