Glass hollow fiber membranes with ultra-micropores (pore diameter less than 1 nm) were prepared by acid leaching. The effects of various acids and leaching times on the preparation of the membranes were investigated. It was found from acid leaching that the sodium ions of the Al-0 (composition: 62.5SiO(2)-28.3B(2)O(3)-9.2Na(2)O wt.%) and Al-3 (composition: 62.5SiO(2)-27.3B(2)O(3)-7.2Na(2)O-3.0Al(2)O(3) wt.%) glass hollow fibers were completely eluted after 10 and 90 min, respectively, with 3 mol/dm(3) HNO3, and after 30 and 90 min, respectively, with 3 mol/dm(3) H3PO4 or 3 mol/dm(3) CH3COOH. Ideal separation factors of H-2/N-2 were measured at 373 K using a gas mixture of 50% H-2-50% N-2. The membranes showed highly selective separation of hydrogen. Values for the Al-0 hollow fiber membrane treated with 3 mol/dm(3) HNO3 reached more than 80. However, ideal separation factors of H-2/N-2 for the Al-3 hollow fiber membranes treated with various acids were smaller. This is thought to be due to the addition of Al2O3. Four coordination aluminum species (AlO4-) are strongly attracted to alkali metal ions because of charge compensation. Sodium ions in the Al-3 glass hollow fibers were, therefore, prevented from moving within the glass network and the destruction of the glass network during acid leaching caused defects in the Al-3 glass hollow fiber membranes. The formation mechanism of the ultra-micropores was also investigated. From N-2 adsorption measurement and gas separation measurement, it was concluded that the ultra-micropores of the membranes were formed by elution of the phase-separated sodium borate-rich phase at sub-nanometer level and that these defect-free membranes could only be obtained by using phase separation rather than elution of homogeneously distributed ions from homogeneous glasses (no phase separation) such as the alkali-silicate glasses reported previously.