The increase of the rebars corrosion rate due to the concrete carbonation is the major cause of reinforced concrete degradation. The aim of this study was to investigate the transition from passive to active corrosion of mild steel rebars in carbonated concrete. For this purpose, electrochemical techniques (polarization curves, free corrosion potential measurements) and surface analyses (EDS, XRD, XPS) were used. Five different electrolytes, with pH ranging from 13 to 8.3, were chosen to simulate the interstitial concrete pore water at various degrees of carbonation. The results indicate that the transition pH is between 10 and 9.4. XPS results indicate a passivation of mild steel for pH values ranging from 13 to 10 due to the formation of a thin iron III oxide layer. Immersion tests highlight the importance of the buffering effect of the carbonate content. At the free corrosion potential in an aerated solution, a decrease of the carbonate content increases the corrosion rate. On the opposite, at low electrode potential, the kinetics of oxidation increases with the carbonate content. (C) 2005 Elsevier Ltd. All fights reserved.