Glass hollow fiber membranes with ultra-micropores (pore diameter less than 1nm) 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: 62.5SiO₂28.3B₂O₃-9.2Na₂O wt.%) and Al-3 (composition: 62.5SiO₂27.3B₂O₃-7.2Na₂O-3.0Al₂O₃ wt.%) glass hollow fibers were completely eluted after 10 and 90min, respectively, with 3mol/dm³HNO₃, and after 30 and 90min, respectively, with 3mol/dm³H₃PO₄ or 3mol/dm³ CH₃COOH. Ideal separation factors of H₂/N₂ were measured at 373K using a gas mixture of 50% H₂-50% N₂. The membranes showed highly selective separation of hydrogen. Values for the Al-0 hollow fiber membrane treated with 3mol/dm³HNO₃ reached more than 80. However, ideal separation factors of H₂/N₂ for the Al-3 hollow fiber membranes treated with various acids were smaller. This is thought to be due to the addition of Al₂O₃. For 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 N₂ 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. Copyright (C) 2000 Elsevier Science B.V.