The hydrophobic nature of a commercial sample of dealuminated zeolite Y (DAY), heretofore referred to as Y901 and available as CBV901 from Zeolyst International, has been examined. Studies were focused on Y901 and related zeolites that were interfaced with bathing polar solvents. Binding isotherms for uptake of non-polar arene test solutes by various zeolites were evaluated from measurements of solute depletion in test solutions that were stirred with zeolite powders. The affinity of naphthalene (NP) for Y901 was found to increase with increasing water content in methanol–water (M/W) mixtures. Up to 2 NP per supercage could be loaded into Y901 from 50/50M/W (v/v). No affinity of NP for Y901 could be detected in neat MeOH. A similar response to water content was observed for uptake of anthracene (AN) and pyrene (PY) by Y901, although studies with these probes were more limited compared to NP due to their inherently lower solubility in polar solvents. In 50/50M/W, the NP-Y901 binding isotherm was sigmoidal in shape. The sigmoidal shape was explained by dimer formation, or preference of NP for NP-occupied supercages, and is supported from observations of static excimer fluorescence at low NP loadings not expected to produce double-occupation of NP in supercages. No affinity of NP for either NaX or NaY could be detected in any polar solvent, regardless of water content. Pyrene excimer formation was also examined in solvated Y901 and was found to be essentially uninhibited in a variety of solvents. In contrast, excimer formation was strongly inhibited in solvated NaX and NaY. This finding lends support to the conclusion from a previous study that the polar-solvated interior of Y901 is less rigid or more fluid than that of NaX and NaY. Such fluidity is likely to be an important factor in the kinetics of solute uptake by Y901. The thermodynamic driving force for uptake of non-polar solutes by Y901 can be explained by the hydrophobic effect and is similar to that found in reversed-phase liquid chromatography. The interior of Y901 is viewed by the solute as an essentially non-polar medium, regardless of the water content of the bathing solvent. The current study reinforces previous assertions from photophysical studies carried out in this laboratory concerning the polarity and fluidity of the environment encountered by solutes in polar-solvated Y901. It also comments uniquely on the hydrophobicity of Y901 from studies of adsorption from solution. Previous descriptions of the hydrophobicity of Y901, and DAY in general, have been based mainly on studies of adsorption from the gas phase.