An industrial V-P-O catalyst (DuPont), comprising vanadyl pyrophosphate crystallites encapsulated by a silica shell, has been investigated. Reaction evaluation for butane oxidation was conducted over (i) the catalyst precursor, (ii) the calcined precursor and (iii) equilibrated material (2 years on-line) in a fixed-bed reactor. Characterization was conducted on these three samples by XRD, SEM, TEM and XPS. The core-shell particles have also been examined with a novel X-ray ultramicroscopy (XuM) technique which reveals the internal structure without having to section the material. The microstructural and surface compositional changes that have occurred over the course of its industrial lifespan in a fluidized bed reactor are correlated with the catalytic activity measurements. (C) 2009 Elsevier B.V. All rights reserved.