Co3O4/La-modified-CeO2/Al2O3 powders washcoated onto ceramic and metallic monolithic catalysts have been compared in order to evaluate the contribution of both the type of substrate and the total washcoat loading on the actual surface structure of the corresponding monolithic devices. Detailed structural and chemical characterization studies have been performed combining FIB (Focused Ion Beam) and STEM (Scanning Transmission Electron Microscopy). Atomic scale STEM data recorded on the electron transparent lamellae prepared by FIB were complemented with those coming from micron scale analysis performed by different SEM (Scanning Electron Microscopy) techniques combined with digital image analysis. X-Ray Energy Dispersive Spectroscopy (X-EDS) elemental maps, acquired from the FIB lamellae, revealed the occurrence of an unexpected surface structure with large ceria aggregates constituted by crystallites with an average size 5 nm and Co3O4 nanocrystallites with two layered structure underneath of some of these aggregates. The existence of this particular structure has been observed for all the samples analyzed here, and seems to be independent of either the nature of the support (ceramic or metallic) and/or the total amount of deposited material. All the investigated monolithic catalysts show high activity and selectivity to total oxidation of model VOCs (Volatile Organic Compounds) such as toluene and ethylacetate. The present study highlights the key role of the combination of the FIB sample preparation and STEM to monitor the influence of the preparation method on the actual surface of catalytic monolithic devices, confirming the occurrence of microstructural features driven by physical processes involved in the multistep procedure of the washcoating technique followed to prepare the final monolithic device. (C) 2015 Elsevier B.V. All rights reserved.