Keggin-type P/Mo polyoxometalates (POM) were dispersed in a high-surface-area silica gel by means of a co-gelation procedure, in order to prepare catalysts for the gas-phase oxidation of isobutane to methacrolein and methacrylic acid. The aim was the dilution of the active phase, in order to favor the dispersion of the considerable reaction heat and limit the oxidative degradation of the desired products. However, the diluted heteropolyacid was less stable than the undiluted one, because of the exchange between P and Si as the heteroatom. When the gel was prepared in the presence of tetrapropylammonium hydroxide, the thermal decomposition of the latter generated the ammonium ion that replaced for protons in the cationic position of the POM. This enhanced the structural stability of the diluted POM at 350 degrees C under reaction conditions. However, no improvement of the catalytic performance was obtained for the diluted catalysts as compared to the undiluted systems, because of the combustion of methacrylic acid, of the modification of the POM redox properties and of the considerable extent of POM decomposition. (c) 2007 Elsevier B.V. All rights reserved.