The separation of various gaseous hydrocarbons with the aid of polymer membranes is considered; attention is focused on the separation of gaseous C-1-C-4 alkanes, which are the components of natural gas and associated petroleum gases. It is noted that a practically important property of membrane materials is the thermodynamic selectivity of a membrane, which makes it possible to enrich a permeate with heavier alkanes. Up to this point, polyacetylenes exhibited the best transport parameters for the solution of this problem. The second experimental section of this paper describes a study of the separation of CH4 + C4H10 binary mixtures on films based on additive poly[3-(trimethylsilyl)tricyclononene-7]. It is demonstrated that this highly permeable saturated glassy polymer exhibits thermodynamic selectivity in experiments with both the individual gases (CH4 and C4H10) and their mixtures and provides fivefold butane enrichment of a permeate. The test polymer and others additive Si-containing norbornene polymers are of interest as membrane materials for the separation of hydrocarbon gases.