Untreated USY zeolites have low tolerance to hot liquid water, which makes them unsuitable for upgrading of biomass-derived liquid compounds in biphasic systems. However, the stability of USY zeolites can be greatly improved by functionalizing their external surface with organosilanes. Here, the structural stability of a commercial H-USY zeolite (Si/Al = 30) in the presence of hot liquid water has been investigated after functionalization with organosilanes of varying alkyl chain length (C2-C18). The structural tolerance of various samples to liquid water has been compared upon exposure to reaction conditions in water/oil emulsions at 200 C. In another set of experiments, structural changes have been monitored after exposure to increasing partial pressures of pure water vapor until a liquid phase is formed. The losses in crystallinity, surface area, and microporosity have been quantified by N-2 physisorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and infrared spectroscopy (DRIFTS). It is shown that the rate of collapse of the crystalline structure is not determined by the chemical potential of the water molecule inside the zeolite, but rather by the presence of a liquid aqueous phase, which favors dissolution and mobility of ions. Therefore, the zeolite deconstruction under hot liquid water can be described as a phenomenon that is more similar to the recrystallization that occurs under synthesis conditions than to a chemical attack, which explains the stability improvement obtained upon hydrophobization. Finally, the activity losses of the different zeolites in the presence of water have been evaluated by using the alkylation of m-cresol in a liquid biphasic system as a probe reaction, which is relevant to biomass conversion processes. (C) 2013 Elsevier Inc. All rights reserved.