AbstractPolymers of norbornenes with SiMe3and Si(Me2)CH2SiMe3groups were synthesized and mass transfer properties of the polymers were studied. Ring‐opening polymerizations of these monomers were realized in the presence of homogeneous WCl6‐based and heterogeneous Re2O7‐based metathesis catalysts. Introduction of silicon‐containing groups in the polynorbornene main chain results in a drastic increase of the permeability (P) and diffusion (D) coefficients for different biatomic and multiatomic gases. Comparison of polymers having SiMe3and Si(Me2)CH2SiMe3substituents shows lower permeability and diffusion coefficients in the latter case. The combination of transport properties (Pivalues) and separation factorsaij=Pi/Pjof the two new silicon‐containing polynorbornenes permits one to consider them as representatives of the most permeable and sufficiently permselective group of polymers. Transport properties of the polymers studied were analysed on the basis of the free volume