Gas permeation, sorption, and free volume was studied in a series of glassy vinylic type Si-containing polymers. They included p-substituted derivatives of polystyrene, which were compared with polystyrene, and poly(vinylphenyldimethyl silane). The strongest increase in permeability and diffusion coefficients were observed for the polymer containing the SI(CH3)(3) group directly attached to phenylene ring. An introduction of spacers between phenylene ring and Si(CPI3)(3) group results in much lower gas permeability. Positron annihilation study of free volume in these polymers showed that lifetime spectra are composed of four components. It implies that size distribution of free volume in these polymers, which are characterized by a modest level of gas permeability, is bimodal, a feature previously observed only for extra large free volume and permeability materials such as poly(trimethylsilyl propyne).