Abstract: A hollow, porous, silica-rich fiber, a process for producing same, and a process for enriching at least one gas from a gaseous mixture utilizing at least one fiber to result in gas separations with good permeability and good selectivity. The fiber is non-crystalline and has pore sizes having a range of 1 to around 50 Angstroms in diameter and a mean pore size of around less than 20 Angstroms in diameter and a filament diameter in the range of 1 to around 250 microns and a wall thickness in the range of around 1 to 50 microns. The fibers with a fine pore structure and thin walls are produced by forming hollow glass fibers hindered or fully phase-separable boron and/or alkali metal glass composition, by attenuation from melt at speeds in the range of 500 ft/min to around 30,000 ft/min. The phase-separable glass fibers are heat treated to yield limited phase-separated hollow glass fibers and these fibers are leached to extract acid and/or water soluble components.