The atomic structure of oxide films formed on carbon steel that are exposed to highly alkaline simulated concrete pore solutions was investigated using Electron Energy Loss Spectroscopy (EELS). In particular, the effect of chloride exposure on film structure was studied in two types of simulated pore solutions: saturated calcium hydroxide (CH) and a solution prepared to represent typical concrete pore solutions (CP). It was shown that the films that form on carbon steel in simulated concrete pore solutions contained three indistinct layers. The inner oxide film had a structure similar to that of (FeO)-O-II, which is known to be unstable in the presence of chlorides. The outer oxide film mainly resembled Fe3O4 ((FeO)-O-II center dot(Fe2O3)-O-III) in the CH solution and alpha-(Fe2O3)-O-III/Fe3O4 in the CP solution. The composition of the transition layer between the inner and outer layers of the oxide film was mainly composed of Fe3O4 ((FeO)-O-II center dot(Fe2O3)-O-III). In the presence of chloride, the relative amount of the Fe-III/Fe-II increased, confirming that chlorides induce valence state transformation of oxides from Fe-II to Fe-III, and the difference between the atomic structures of oxide film layers diminished. (C) 2013 Elsevier B.V. All rights reserved.