The solid-state behavior of (NH4)(4)[(PMo11VO40)-O-V], crystallized from a water/dioxane solution, has been studied. Decomposition of one of the four ammonium cations occurs easily and a thermal treatment in air at 220 degreesC leads to (NH4)(3)H[PMo11VO40]. Further ammonium decomposition was observed between 220 and 300 degreesC up to (NH4)(0.8)H-3.2[PMo11VO40]. Only one cubic phase was evident and the lattice parameter increases as ammonia is eliminated. Rietveld refinement of X-ray patterns of all samples is better with an occupancy of 3 for anionic sites, as previously proposed for Cs-4[PMo11VO40]. Solid-state substitution of cesium for ammonium cations has been performed by thermal treatments at 300 degreesC of the ammonium salt impregnated by cesium nitrate. Only three ammonium cations can be substituted. The catalytic behavior of these salts for the oxidative dehydrogenation of isobutyric acid reveals high catalytic activity and selectivity to methacrylic acid for all the samples up to three cesium cations but fall down hereafter. (C) 2003 Elsevier Inc. All rights reserved.