The influence of commercial superplasticizers on the mechanism of corrosion of steel reinforcements embedded in concrete has been investigated by the impedance spectroscopy technique using Portland cement mortars with and without added chloride. Significant differences in spectra in the range 0.03-65 kHz are shown to occur as a function of composition and moist cure duration up to 240 days. The impedance spectra in the early stages indicate surface layer formation, attributed to superplasticizer adsorption to the surface of reinforcing rods. The adsorbed surface layer persists only within the initial 90 day cure. At longer ages, the superplasticizer layer breaks down. However, calcium lignosulphonate gives markedly lowered corrosion rates in the presence of chloride; the other plasticizers corrode at essentially the same rate as the control without plasticizer.