In this research, a mechanistic approach was used to investigate the effect of molybdate ion on the critical pitting temperature (CPT) of 2205 duplex stainless steel. Firstly, the CPT of 2205 DSS was measured using potentiodynamic and potentiostatic polarization. It was found that with addition of 0.0005, 0.005 and 0.05 M Na2MoO4 to 0.5 M NaCl solution, the CPT increases approximately 4, 9, and 14 degrees C, respectively. Using the lead-in pencil electrode technique, the mechanism by which molybdate ion influences the CPT was interpreted using the CPT model proposed by Salinas-Bravo and Newman. The results showed that molybdate has a negligible effect on the pit solution chemistry, resulting in a slight change in the diffusion-limited current density. However, it reduces the rate of alloy dissolution within the simulated pit solution, which was found as a reduced maximum current density. (c) 2019 The Electrochemical Society.