A thin SiOx surface layer was formed on porous Nylon(R) membranes coated with a cross-linked polysiloxane by exposure to ultraviolet light at room temperature in the presence of atmospheric oxygen. The transformation of the cross-linked polysiloxane to SiOx was monitored by X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy (ATR-IR), contact angle analysis and atomic force microscopy (AFM). A continuous surface layer of SiOx formed after about 1 h of UV exposure. AFM analysis revealed that a nodular structure with a characteristic size less than 0.5 nm formed after extended UV exposure. Gas permeation measurements on the composite membranes documented interesting gas separation properties: the CO2 permeance of the cross-linked polysiloxane/Nylon(R) composite membrane before irradiation was 8.9x10(-8) mol m(-2) s(-1) Pa-1, and the CO2/N-2 selectivity was 22; after UV-ozone induced formation of the SiOx surface layer, the permeance decreased to 5.1x10(-8) mol m(-2) s(-1) Pa-1 while the selectivity for CO2/N-2 increased to 48.