The oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA) was studied over the heteropolyacid H4PVMo11O40 and the acidic salts NaxH4-xPVMo11O40 and CuyH4-2yPVMO11O40, supported on silica, as catalysts. The presence of copper as counter-cations in the heteropoly compounds increases the activity and the selectivity to MAA. This increase is probably due to the simultaneous presence in the solids of Bronsted acidity and oxygen vacancies with an adequate ratio. These latter sites can more easily be obtained in the presence of Cu2+ and V4+ than with Na+ counter-ions in the catalysts.