Oxidative dehydrogenation of isobutyric acid to methacrylic acid has been studied using Cs salts of 12-molybdovanadophosphoric acids as catalysts (CsnH3+x-nPMo12-xVxO40, n = 0-(3 + x), x = 0-2, which will be abbreviated as CsnMo12-xVx). Among them, Cs2.75Mo11V showed remarkably high performance; the hi hest conversion (97%) and selectivity for methacrylic acid (MAA, 78%) at 623 K. At the same reaction conditions, Mo11V (i.e. n = 0, x = 1) gave only 40% conversion and 52% selectivity, As for the series of Cs2.75Mo12-xVx, the activity was in the order of x = 1 > x = 2 > x = 0 and the MAA selectivity, x = 1 greater than or equal to x = 2 > is = 0. The nature of Mo11V was confirmed by IR to be partially destroyed during pretreatment in O-2 at 623 K, while Cs2.75Mo11V decomposed only slightly, This fact indicates that the thermal stability afforded this catalyst to make efficient use of the redox ability controlled by incorporation of V, and the enhanced redox ability, high surface area, and high surface acidity of Cs salts would be the main reasons for their effectiveness.