The microstructural development of Mn1.5Co1.5O4-coated Crofer22 APU has been studied using cross-sectional transmission electron microscopy. Alloy samples were coated via a slurry process involving consolidation by reduction and re-oxidation, and these samples were then oxidized at 800 degrees C for times of up to 1000 h. All samples exhibited a thin chromia scale at the alloy/coating interface plus spinel phases as a reaction layer between the chromia and the manganese cobaltite coating. The oxidized samples also exhibited pockets of stoichiometric MnCr2O4 spinel at the chromia/alloy interface and internal Ti-rich oxides in the alloy below the chromia. The reaction layer spinels exhibit remarkable changes in thickness, morphology and composition, and these effects are explained on the basis of changes in the diffusive fluxes during the different stages of coating application and subsequent exposure. The possible consequences of these observations for the degradation mechanisms that could affect SOFC interconnects produced from MCO-coated Crofer22 APU are discussed. (C) 2012 United Technologies Corporation. Published by Elsevier B.V. All rights reserved.