The hydrophobic character of three dealuminated mordenites has been investigated and quantified from the kinetic study of the hydroxymethylation of furfuryl alcohol with aqueous formaldehyde. The affinity constant of the substrate for the siliceous framework of the zeolite reflects this hydrophobic character and increases with the Si/Al ratio. These results are in agreement with adsorption measurements of furfuryl alcohol from aqueous solutions on dealuminated mordenites.