An anomalous oxide scale formed on Fe-Cr alloy interconnects was investigated under operation conditions for solid oxide fuel cells (SOFCs). A dilute CH4-H2O was flowed one side and air was flowed the other side under constant current flow (0.3A cm(-2)). The Pt-mesh/alloy interface resistance in CH4-H2O increases with reaction time in a parabolic relationship. The diffusion controlled mechanism was proposed for the Pt-mesh/alloy interfaces with "normal" oxide scale. An "anomalous" thick oxide scale was found near the outer of glass/alloy interfaces, which was exposed to air. The thick oxide scale was composed of Fe-oxide in the outer layer and Cr-oxide in the inner layer (above +/- 50 mu m thick from the original level) with SiO2 and Al2O3 inner oxides. The microstructures and elemental distribution of the anomalous thick oxides were investigated. A possible formation mechanism of the anomalous oxide scale was discussed by considering the reaction with glass components. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.