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Fluorinated polymers have a set of unique properties, including improved chemical stability and thermal stability and good barrier and membrane parameters, which are mainly defined by their surface properties. This article presents systematic data on the direct fluorination of the polyimide Matrimid ® 5218, a commercially available polymer suitable for the formation of gas-separation hollow fibers. Changing the fluorination conditions (i.e., the fluorinated mixture composition, fluorine partial pressure, and treatment duration) allows the rate of formation of the surface-fluorinated layer over the 0.1-10 m range to be kept under control. The physicochemical properties of modified layers (i.e., the chemical composition, formation of radicals, refractive index, IR and UV spectra, density, and surface energy) are examined. The thickness of the fluorinated layer (␦ F ) depends on the fluorination duration (t): ␦ F ϳ t 0.5 . During fluorination, hydrogen atoms are replaced with fluorine, double bonds are saturated with fluorine, and at least one CN bond in the five-member ring is disrupted. Fluorination results in a significant increase in the polymer density, transparency in the visible and ultraviolet regions of spectra, and a reduction of the refractive index. A high concentration of long-living radicals (up to ϳ5 ϫ 10 19 radicals/cm 3 of the fluorinated layer) is generated under fluorination. This can be used for subsequent grafting (e.g., with acrylonitrile).