The results of one-sided treatment of polyvinyltrimethylsilane (PVTMS) films using low-temperature air plasma are presented. The treatment was carried out at 25°C in air at a pressure in the reaction chamber of 10–25 Pa and a processing time of 10–60 s. The surface of the plasma-treated PVTMS films was analyzed by X-ray photoelectron spectroscopy and atomic force microscopy and the contact angle values determined. The effect of the plasma conditions, such as the treatment time and the pressure in the reactor chamber, on the transport properties of the modified membrane for He, O₂, N₂, CO₂, and CH₄ were investigated in detail. The permeability of all plasma-treated PVTMS films for the studied gases except for He was found to decrease depending on the modification conditions, leading to an increase in selectivity for such pairs as He/CH₄, CO₂/CH₄, and O₂/N₂. The selectivity of the modified PVTMS achieved a maximum value when the films were treated with plasma for 20–40 s at a chamber pressure of 15–20 Pa. The effective diffusion coefficients of the studied gases were obtained experimentally. It was shown that the volumetric diffusion of gases, particularly O₂ and N₂, in PVTMS before as after plasma treatment is described by the classical diffusion equations. The theoretical approaches to the influence of boundary conditions on selective gas transfer through polymeric membrane are considered. To estimate the prospects for the long-term use of membranes based on modified PVTMS, the stability of the parameters gas permeability and selectivity of the modified samples was investigated over 9 months. It was shown that the modification of PVTMS by the method of low-temperature plasma in air is a promising method for expanding the spectrum of membrane processes based on existing commercial membranes.