A new Mo-based structure was detected by high resolution transmission electron microscopy (HRTEM) and electron diffraction techniques in a molybdenum based mixed oxide catalyst after operation in the partial oxidation of acrolein at temperatures between 533 and 573 K for 9 h. This new crystalline phase has an orthorhombic crystal system. The structure has presumably a space group of Pban, but Pmmn cannot be excluded. XRD analysis was further used to refine this new Mo2O5 structure. The lattice constants were estimated to be a = 1217.5, b = 374.7 and c = 404.4 pm, respectively. The elemental composition of the crystal was determined by the EDAX system attached to the electron microscope to be about (Mo0.6V0.3W0.1)(2)O5-x. This new structure is discussed in relation to earlier reports on a comparable composition and with respect to its possible role for partial oxidation catalysis. (C) 2002 Published by Elsevier Science B.V.