Heterogeneity of the thermally oxidized surface of a polycrystalline iron substrate was investigated by using two-dimensional (2D) ellipsometry and a micro-capillary cell (MCC). Thickness mapping of a bi-layered oxide film (inner Fe3O4 layer/outer alpha-Fe2O3 layer) on the iron substrate was performed by 2D ellipsometry, and it was demonstrated that the grain-dependency of the thickness of Fe3O4 determined the heterogeneity of oxide thickness on the polycrystalline iron. Degradation behavior of the thermally oxidized surface during galvanostatic polarization was observed by in situ 2D ellipsometry. The transients of ellipsometric parameters showed different trajectories depending on the substrate orientation, indicating that the magnitude of degradation of the oxide by the cathodic current differed depending on the heterogeneous oxide structure. EIS measurements with the MCC revealed that donor density N-D of the oxide was greater in the order of {0 0 1} > {1 1 1} > {1 0 1}. It was concluded that the grain-dependency of the oxide thickness and N-D was attributed to the anisotropic growth of the two oxide layers. (C) 2016 The Electrochemical Society. All rights reserved.