The effect of sulfate ions on the electrochemical behaviour of iron electrodes in the presence of bicarbonate ions has been investigated. In solutions containing sulfate ions only, iron electrodes experience uniform corrosion and sulfate ions are aggressive. In the presence of bicarbonate ions, the sulfate ions become less aggressive and a pitting-repassivation process is observed. The differences in electrochemical behaviour have been compared and explained by examining the formation of probable products and their physicochemical properties during electrochemical polarization.