Preparation of molybdenyl and vanadyl acidic salts Of [PMo12O40](3-) has been achieved by cationic exchange between solid heteropolyacid and acetylacetonate complexes in toluene. Ammonium vanadyl salts were prepared by solid-state reaction between acidic vanadyl salts and urea at 250 degreesC. Importance of hydration state or exposure to water vapor on the preparation and structure of acidic vanadyl salts was revealed. Solid solution of acidic vanadyl salts with cubic Fd (3) over barm lattice can be obtained. Exposure of ammonium vanadyl salts to water vapor gives rise to a rebuilding of at least the surface of the particles leading to segregation into a pure ammonium Pn (3) over barm phase and a pure highly hydrated vanadyl Fd (3) over barm phase. Catalytic behavior of bulk solids for oxidative dehydrogenation of isobutyric acid at 320 degreesC was determined in presence of water. For ammonium vanadyl salts, the yield in methacrylic acid increases as the proportion of vanadyl phase increases until 0.75 VO2+ per polyanion and then a plateau is reached. This observation is consistent with the characterizations suggesting that these salts are composed of a core of ammonium salt embedded in hydrated vanadyl salt. Presence of molybdenyl in cationic position neither allows an increase of the catalytic performances nor stabilizes the catalysts. (C) 2004 Elsevier B.V. All rights reserved.