Vanadium molybdenum mixed oxides are used for the technical production of acrylic acid by partial oxidation of acrolein. Although this industrial process is established since several years, the catalytic mechanism is not known in detail. For an improved understanding of the reaction on a microscopic scale, we used subsystems such as V-Mo-oxides without additional promoters as model catalyst. Three series of mixed oxides with the chemical composition VxMoyOz, x + y = 1, were prepared; one by melting the pure oxides, one by crystallisation and another by spray-drying of ammonium salt precursors and subsequent calcination. The phases were analysed by X-ray diffraction. Most of the samples consist of two different vanadium molybdenum mixed oxides, a hexagonal h-(VMo)O-3 and a (V,Mo)(2)O-5-phase, which is structurally related to vanadium pentoxide. The stability and the transformation of these phases were also examined. The selectivity and conversion rate for the partial oxidation of acrolein to acrylic acid was determined by temperature programmed reduction (TPR) to optimise the vanadium molybdenum ratio for an efficient catalytic phase. The results indicate a vanadium to molybdenum ratio of 3:7 as the most promising with respect to selectivity and activity at low temperatures. (C) 2004 Elsevier B.V. All rights reserved.