The commonly observed irreversibility of the passive state on carbon steel in alkaline solutions was examined in simulated concrete pore solution under anoxic conditions replicated by applying relatively negative film formation potentials, at which the cathodic process dominates the passive system. The fundamental source of this irreversibility was investigated by describing the kinetics of the passive system via a previously developed mixed potential model that combines the point defect model accounting for the anodic process and the Butler-Volmer equation accounting for the cathodic process. Electrochemical impedance spectroscopy, Mott-Schottky analysis and model optimization were performed at each potential when the potential was first stepped in the anodic direction and then in the cathodic direction. The experiment and optimization results demonstrate that the irreversibility of the passive state is closely associated with the discrepancy in the defect structure of the passive film upon opposite stepping directions of the film formation potential, and is essentially caused by the slower film formation and slower film dissolution during the cathodic potential stepping than those during the anodic potential stepping. (c) 2015 The Electrochemical Society. All rights reserved.