In this research, the effect of thiosulfate ion on pitting corrosion behavior of 316 stainless steel was studied. Part I of this work deals with the influence of thiosulfate on the critical pitting temperature of 316 SS in 0.1 M chloride solution and studies the effect of this ion on the pit chemistry. Electrochemical experiments performed in absence and presence of 0.01 M thiosulfate ion revealed that thiosulfate deteriorates the CPT. Pencil electrode studies showed that thiosulfate addition decreases both saturation concentration of metal cations necessary to metal salt precipitate at the pit bottom (CS) (the value of diffusion controlled limiting current density (i(Lim)) was decreased) and the ratio of C*/C-S (where C* is the critical concentration of metal cations in pit solution essential for a pit to remain stable) while it increases the anodic dissolution rate in a simulated pit solution. Furthermore, scanning electron microscopy showed that the lacy cover formed over the pits mouth has coarser pore structure in the presence of thiosulfate ion. (C) 2014 The Electrochemical Society. All rights reserved.