Molybdovanadophosphate heteropolycompounds, H4PMo11VO40 and HgPMo6V6O40 and the corresponding Cs salts, were synthesized. The influence of thermal treatment on their Keggin primary structures and their acidic properties were studied. The presence of Keggin units and the V incorporation into these units were observed by TGA, DTA, DRS and FT-IR. H4PMo11VO40 possesses an acid strength higher than that of the V-free sample and HgPMo6V6O40, at room temperature. Heating above 200 degreesC leads to considerable structural changes and acidity decrease. The Cs salts from H4PMo11VO40 possesses stronger acidic sites than those from HgPMo6V6O40. Moreover when the Cs number increases, the acidic properties decrease. The salts with one, two, and three Cs are more unstable with temperature than the salt with 2.5 Cs. The specific surface area for the Cs salts depends on the temperature and Cs number. The prepared heteropolycompounds were used as catalysts for the liquid phase reaction of synthesis of 2,6-dimethyl-p-benzoquinone from 2,6-dimethylphenol, using hydrogen peroxide, with high yields. A selectivity to the p-benzoquinone of 79-59% was obtained when the reaction was catalyzed by Cs salts from the H4PMo11VO40 acid. (C) 2004 Elsevier B.V. All rights reserved.