Dense films of poly(vinyltrimethyl silane) (PVTMS) and poly(trimethylsilyl norbornene) (PTMSNB) having different thicknesses in the range l = 5-150 mu m were cast from hydrocarbon solutions. It was shown that a density is inversely proportional to the film thickness. The following equation holds for the density rho : l/rho = l/rho, - b/l. Permeability and diffusion coefficients were determined using the time lag method in respect to different gases. For all the gases, diffusion coefficients decrease when film thickness decreases and film density increases. A correlation of diffusion coefficients with fractional foe volume were demonstrated. On the other hand, permeability coefficients are nearly independent of the thickness and density. Possible mechanisms of this behavior are discussed.