Present paper deals with the use of the electrochemical impedance spectroscopy to identify different processes in the passive layer growth over steel rebar surface immersed in an alkaline media simulating the concrete pore solution. Two cases have been considered: a passive layer spontaneously grown in a high alkaline media and a passive layer assisted by the application of an anodic potential in the same media. The application of electric equivalent circuits allows distinguishing between the different mechanisms occurring in this passive layer when grows in different conditions. An electric equivalent circuit with two RC loops connected in parallel is often used for fitting the EIS diagrams obtained for spontaneous growth of passive layers in the alkaline solution simulating the concrete pores. However, when the passive layer is formed under anodic polarization, a Warburg element must be introduced in the equivalent circuit. According to the Point Defect Model (PDM), this Warburg element is allocated to the transport of oxygen vacancies through the passive layer, which concentration changes with the potential. (c) 2007 Elsevier Ltd. All rights reserved.