The ammonium salt of the 1-vanado-11-molybdophosphate in which vanadium is at the oxidation state IV was synthesized in order to obtain the mixed ammonium-cesium and pure cesium salts by thermal Cs+/NH4+ solid State exchange. Study of its thermal behavior showed that a phase, named phase A, isotype with the cubic phase of (NH4)(3)[PMo12O40] is Obtained and is stable up to 410 degrees C. This phase A contains two different Keggin heteropolyanions, [(PMo11VO40)-O-IV](5-) and [PMo12O40](3-) in equal quantities and, in addition to the ammonium cations, vanadyl cations balance the supplementary negative charges of the vanadium containing heteropolyanion. Its molecular composition is (NH4)(6)(VO)[(PMo11VO40)-O-IV][PMo12O40] This phase has a low surface area (11 m(2)/g), on the contrary of the ammonium cubic phase of (NH4)(3)[PMo12O40] Obtained by precipitation from aqueous solutions of the heteropolyacid. But it is about twice more active in the oxidative dehydrogenation of isobutyric acid and more selective to methacrylic acid, as it is expected from previous studies on vanadyl salts. Thermal Cs+/NH4+ exchanges performed on the phase A are not complete and cannot be carried out without structural modifications. (C) 2000 Elsevier Science B.V. All rights reserved.