The use of stainless steels as concrete reinforcement is becoming increasingly popular in coastal and marine constructions in order to prevent corrosion induced by chloride ions penetrating into the concrete. In these highly aggressive situations, the lean duplex stainless steels have been extensively employed due to their high mechanical and corrosion resistances. However, the influence of Mo addition on pitting corrosion resistance of these steels is not clearly understood in alkaline chloride conditions even if this element is widely associated to an increasing corrosion resistance in acidic and neutral environments. Therefore, understanding Mo mechanism on corrosion resistance in alkaline media is hence of major importance to the setting of optimized alloy composition. The present work aims to study the effect of Mo addition on pitting corrosion properties of lean duplex stainless steels in alkaline environments (concrete). The results are discussed with respect to the influence of Mo addition on pitting potential for two industrial duplex alloys (1.4362 and 1.4462) in several aggressive media mainly in synthetic, chlorinated and carbonated solution simulating concrete pore environments (pH10 solution with carbonates and chlorides ions). In order to establish the real role of Mo addition on lean duplex corrosion and passivation properties, the corrosion behaviors of two specific laboratory lean duplex alloys, which the only difference between then is the amount of Mo (0 and 3% wt. Mo), are also studied. Furthermore, scanning electronic microscopy (SEM) was used to verify which phase of duplex microstructure (ferritic or austenitic) is mostly susceptible to the pit nucleation in these corrosion conditions as the role of Mo is probably different to each simple phase.