The catalytic behavior of molybdophosphoric or tungstophosphoric acids supported on different silica-aluminas, obtained by sol-gel method, in a hydroxyl protection reaction, phenol tetrahydropyranylation, was studied. Different techniques were used to synthesize the supports. In one of them, commercial alumina was added to a tetraethyl orthosilicate (TEOS) solution and then this was hydrolyzed. In the other preparations, TEOS and aluminum tri-sec-butoxide were hydrolyzed, simultaneously or with TEOS prehydrolysis. The catalysts were prepared by incipient wetness impregnation and, after their leaching with toluene, different thermal treatments were applied. The specific surface area of the solids was measured, and the nature of the Keggin heteropolyacid species present in the catalysts was characterized by diffuse reflectance spectroscopy and X-ray diffraction. In addition, their acidic properties were evaluated by potentiometric titration with n-butylamine. The acetal yield was strongly dependent on the catalyst properties, mainly their acidic characteristics which, in turn, depended on the support synthesis. Moreover, the decrease of the catalyst water content through treatment up to 250 degreesC led to higher yields, due to the presence of water in the reaction medium favors the hydrolysis of the acetal. (C) 2003 Elsevier B.V All rights reserved.