Several de-aluminated H-mordenites, prepared by treating a commercial H-mordenite (HM-16, Engelhard) with aqueous HCI solutions, were characterized and tested as catalysts in the liquid-phase hydroxymethylation of guaiacol with aqueous solutions of formalin, for the production of p-vanillic alcohol (3-methoxy-4-hydroxybenzyl alcohol), the intermediate in vanillin synthesis. Samples were obtained with Si/Al atomic ratios from 10 to 36; their acidic and hydrophilic properties have been characterized by means of FTIR spectroscopy of adsorbed probe molecules, namely CO at nominal 77 K, NH3 and H2O at room temperature, and ammonia TPD. De-alumination led to the development of a mesoporous structure, and to a partial structure degradation with the most de-aluminated sample (Si/Al = 36). As to the acidic properties, both Bronsted species in the main channels and in side-pockets were removed, and less acidic hydroxyls formed. The catalytic performance was affected mainly by samples hydrophilicity that considerably decreased upon de-alumination. In the Si/Al ratio range investigated, a decrease in the number of acidic sites led to a complex catalytic behaviour, with a maximum activity for intermediate Si/Al ratio (=25). A higher concentration of aromatic compounds in the pores of more hydrophobic zeolites (higher Si/Al ratios) also led to a higher selectivity towards heavy, di-arylic by-products. A comparison with commercial H-mordenites having similar SUM ratio evidenced that the occurrence of mesoporosity, in strongly de-aluminated samples, also affected the catalytic performance, favouring the formation of bulky di-aryl compounds and leading to lower guaiacol conversion. (c) 2006 Elsevier B.V. All rights reserved.