The role of hydrophobic effects of dealuminated mordenites (Si/Al ratios varying from 7 to 100) has been investigated in two reactions carried out in aqueous/alcoholic media: hydroxymethylation of furfuryl alcohol with aqueous formaldehyde and hydration of phenyl-acetylene in alcohol medium. For both reactions, initial rates and turn over frequencies (TOF) increase in a significant extent with increasing Si/Al ratios. It is shown that the differences thus observed in the TOF values are characteristic of variations in the hydrophobicity of the various dealuminated mordenites, which is confirmed by the corresponding increase of the hydrophobicity parameter h, determined by thermogravimetric analysis of such samples. The hydrophobic character of the zeolites is also discussed in terms of the softness and activity of the corresponding Brönsted acid sites.