Molybdosilicic acid solutions acidified with HCl were impregnated on carbon and silica by the equilibrium adsorption technique. From the equilibrium adsorption isotherms performed at 20 degreesC, it was established that the heteropolyacid amount adsorbed on carbon is higher than that on silica. Differences in shape and initial slope of the isotherm were observed. The interaction of the molybdosilicate anion was higher with carbon surface groups than with silanol groups. The physicochemical characterization of the solids has shown that the molybdosilicic acid presents an undegraded Keggin structure in the supported samples up to 250 degreesC. The surface acidity, mainly related to heteropolyacid adsorbed amount, was higher for samples on carbon. However, the silica-supported acid showed a bulk acidity higher than that on carbon, due to its lower interaction degree.