The hydroxymethylation of guaiacol to vanillic alcohols, which is an important step in the synthesis of vanillin from phenol, has been studied over commercial H-mordenites having three different Si/Al ratios, with the aim to establish relationships between catalytic performance and zeolite chemical-physical features. Catalysts have been characterized following adsorption and desorption of NH3 and H2O by means of both TPD and FT-IR spectroscopy, hence gathering information on the nature and abundance of acidic sites and the hydrophilic/hydrophobic behaviour of the solids. In the adopted range of SUM ratio, between 10 and 58, an increase in concentration of acid sites corresponds to a decrease in catalytic activity. The zeolite property mainly affecting catalytic performance is hydrophobicity, which controls the relative concentrations of reactants in the porous structure, finally affecting the conversion of guaiacol and the distribution of products. The sample having intermediate Si/Al ratio = 23 represents the best compromise between a high conversion of the aromatic, which is proportional to the Si/Al ratio, and a low selectivity to diarylcompounds, undesired by-products of the reaction. The latter are preferentially formed with respect to vanillols and to monoaryl by-products in more hydrophobic H-mordenites, having higher SUM ratio. (C) 2004 Elsevier B.V. All rights reserved.