This work evaluates a new eco-friendly strategy for preparing ultrathin gas selective membranes on top of a microporous support. The method involves the dissolution of small amounts of fluorinated oligomers with alkoxysilane functional groups in supercritical CO2 (scCO(2)) and their transport to the substrate followed by the subsequent deposition/filtration under high pressure using a MFI zeolite membrane support (silicalite-1 (S-1), channel size similar to 0.55 nm. During the deposition process, the oligomers are compressed on the zeolite surface and potentially forced in the intercrystalline defects of the zeolite membrane, if any. The performance of this new type of polymer/zeolite composite membranes has been evaluated for both single gas permeation and gas mixture separations. Attractive results were obtained applying oligomers with short molecular chains (similar to 1.2-2 nm; similar to 300-600 g mol(-1)), easily forming an interpenetrated compact network during the deposition process at Delta P=6 MPa and 50 degrees C. High permselectivities were obtained at 25 degrees C (alpha(He/N2)* = 85-135 and alpha(CO2/N2)* = 50-80 with the He and CO2 permeance in the range 1-2.7*10(-8) mol m(-2) s(-1) Pa-1) together with attractive separation factors (F-He/N2 = 49 and F-CO2/N2 =18). (C) 2012 Elsevier B.V. All rights reserved.