Solvent clustering in membranes, if it occurs, could modify the sorption and diffusion behaviors of polymer membranes and therefore their transport properties. Solvent cluster formations in hydrophilic poly(vinyl alcohol) (PVA), in hydrophobic poly(dimethylsiloxane) (PDMS) and intermediate poly(vinyl acetate) (PVAc) membranes were studied by infrared spectroscopy, differential scanning calorimetry, X-ray scattering techniques, and by visual observations. A water saturated PVAc membrane contains non-freezable (bound) and liquid-like water molecules in equivalent amounts (ca. 1.6 wt%); membrane infrared studies during the sorption process indicated that water penetrates first in the membrane as monomeric molecules which aggregate later to form clusters. In PVA membranes, water molecules can exist under three states, i.e. non-freezable, freezable bound and liquid-like states, depending on the water content in the membrane. The cluster formation seems to affect the membrane selectivity towards water and the water and ethanol diffusivities in PVA membranes. In PDMS membranes, solvent clusters were evidenced by infrared spectroscopy and visual observations. The clustering of alcohols in PDMS makes the sorption behavior of these solvents in the membrane deviate from the Flory-type behavior observed for other solvents : the sorbed amount increases drastically with the surrounding-solvent activity in the high activity range, but the solvent diffusivity decreases.