Sorption/desorption kinetics and sorption equilibria have been determined for ethanol, toluene, and xylene vapors in a poly (dimethylsiloxane) membrane containing about 32 wt % silica resin at 25-degrees-C. Dependence of diffusion coefficient on vapor activity and sorption isotherms have been compared to identify the transport mechanisms of those penetrants in the PDMS membrane. The analysis of Zimm-Lundberg clustering functions showed that all three penetrants had a tendency to form clusters and ethanol molecules might be immobilized by the residual silanol groups within the silica resin in the membrane. The diffusion coefficient of toluene was roughly constant and that of xylene slightly decreased as increasing the vapor activity due to the competing effects of penetrant clustering and solvent swelling of polymer. The diffusion coefficient of ethanol versus activity exhibited a maximum due to the effects of ethanol immobilization and cluster formation. The free-volume effect by solvent swelling to diffusion was obscured by either penetrant clustering or immobilization for the three penetrant-polymer systems.