Composite membranes prepared from mixtures of 3-glycidoxypropyl trimethoxysilane (GPTMS) and different diamines containing polyether segments were synthesized by the sol-gel process. The membranes were obtained by coating asymmetric porous polyacrylonitrile (PAN) supports with the silane solutions. The composite membranes were characterized by single and mixed gas permeation and by atomic force microscopy. The selectivity increased by increasing the molecular weight of the poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol)bis(2-aminopropyl ether) (PAPE), and also with the addition of poly(ethylene glycol) (PEG) to the coating solution. The CO₂/N₂ selectivity values up to 75 and the CO₂/CH₄ selectivity values up to 20 were measured. High gas selectivity was also confirmed by measurements with mixed gas feed, although slightly lower than in the measurements with single gases.