Corrosion processes affecting steel reinforcements embedded in concrete have been investigated by ac impedance spectroscopy over the frequency range 003 Hz to 103 kHz, using mild steel rods embedded in ordinary Portland cement paste. Corrosion of the reinforcing rods was artificially accelerated by adding 5%NaCl (by weight of cement) to the initial mix and the specimens aged either at 40°C and 100%RH or at ∼20°C in a C02 rich atmosphere at 90%RH. The impedance characteristics prior to and during corrosion were monitored. A correlation is made between the impedance results and the rate of corrosion of the rods obtained by the linear polarisation method. The early stage impedance spectra are dominated by charge transfer controlled kinetics which undergo a transition to diffusion control after prolonged aging. The transition to diffusion control occurs after 90 days’ exposure in the carbonated samples, and is marked by the emergence of an intermediate frequency arc, indicative of the formation of a surface layer on the rods. The relative effects of different environments on the principal stages of the corrosion process are evaluated.