In this study, a dilute mixture of volatile organic compounds in aqueous solution was separated by means of hydrophobic pervaporation, using three different poly(octylmethyl siloxane) membranes of various thicknesses. The permeabilities were determined for homologous series of compounds from the chemical groups of alcohols, esters, and aldehydes, and they were then related to some characteristic properties of the permeants. The molar volume, the activity coefficient at infinite dilution in water, and the ability to form hydrogen bonds could explain the resulting permeability, when comparing the values of the different permeants. A critical molar volume was identified, above which the permeability started to decrease drastically with increase in the permeant molar volume as the mobility of the permeant was hindered sterically by the distance between the polymer chains of the membrane. This critical molar volume was related to the degree of crosslinking of the selective layer. Thus, it could be concluded that membranes should be designed with a low degree of crosslinking to obtain better separation properties, especially for larger permeants. From the results, it could also be concluded that the porous support layer could affect the separation properties significantly.