Hydrophobic composite membranes with a crosslinked poly(dimethylsiloxane)-poly(methyl hydrogen siloxane) selective layer were prepared by using a new laboratory made catalyst agent. The pervaporation separation of five organic solvent-water mixtures was carried out with these composite membranes, together with swelling experiments in the same feed mixtures. The volatile organic compounds employed were ethanol, methanol, 1-butanol, acetone, and ethyl acetate. The pervaporation and swelling experiments revealed that both the 1-butanol and the ethyl acetate solutions showed the highest affinity for the composite membrane. When these components were employed as feed solutions, the membranes showed both high selectivity and high permeation. Mechanical-dynamical experiments of swollen and nonswollen composite membranes were also performed. The relaxation spectra were analyzed in terms of the interaction of the components of the different mixtures with the composite membrane, and the free volume corresponding to the each sample was obtained. Once the membranes had reached an equilibrium swelling, a decrease in the free volume was observed.