A thin SiOx selective surface layer was formed on a series of cross-linked poly(dimethylsiloxane) (PDMS) membranes by exposure to ultraviolet light at room temperature in the presence of ozone. The conversion of the cross-linked polysiloxane to SiOx was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) rnicroanalysis, contact angle analysis, and atomic force microscopy (AFM). The conversion of the cross-linked polysiloxane to SiOx increased with UV-ozone exposure time and cross-linking agent content, and the surface possesses highest conversion. The formation of a SiOx layer increased surface roughness, but it decreased water contact angle. Gas permeation measurements on the UV-ozone exposure PDMS membranes documented interesting gas separation properties: the O-2 permeability of the cross-linked PDMS membrane before UV-ozone exposure was 777 barrer, and the O-2/N-2 selectivity was 1.9 after UV-ozone exposure, the permeability decreased to 127 barrer while the selectivity increased to 5.4. The free volume depth profile of the SiOx layer was investigated by novel slow positron beam. The results Show that free volume size increased with the depth, yet the degree of siloxane conversion to SiOx, does not affect the amount of free volume.