Effects of varying concentration of sulphate (SO42-) ion on the pitting behavior of 316SS have been investigated using potentiostatic critical pitting temperature (CPT) measurements, potentiostatic current transient technique and scanning electron microscopy in NaCl solution containing 0.5% Cl- ions. The results demonstrated that when the concentration Of SO42- ion is less than 0.42%, the CPT is surprisingly lower than that without SO42- ion, showing an accelerating effect of the SO42- ion on pit initiation, which is different from the traditional concept. As the concentration of SO42- ion increases beyond 0.42%, the CPT is higher than that without SO42- ion, displaying an inhibiting effect of the SO42- ion on pit initiation. Based on the above results, a qualitative model is proposed to explain the inhibiting and accelerating effect Of SO42- ion on the pit initiation using the mechanism of ions-competitive adsorption between SO42- and Cl- ions. The electric charges calculated in the process of pitting corrosion indicated that the pit morphology and its dimension are dependent on the content Of SO42- ion in chloride-containing solutions. The higher the concentration Of SO42- ion, the larger the dimension of the pit, reflecting an accelerating effect on pit growth. (c) 2007 Elsevier B.V. All rights reserved.