Sodium oxide, Na2O, reversibly absorbs hydrogen, H-2, to form sodium hydride, NaH, and sodium hydroxide, NaOH, which possesses a potential to find its application to reversible hydrogen storage. X-ray diffraction measurements illustrate reversible phase and composition changes during the hydrogen absorption and desorption in the Na-O-H system. Pressure-composition (P-C) isotherm and thermogravimetric (TG) measurements exhibit a hydrogen capacity up to 3.0 wt.%. Temperature-programmed desorption (TPD) measurements demonstrate that mixing NaOH to NaH significantly shifts the hydrogen desorption of NaH towards lower temperature, to which TiCl3 and SiO2 show catalytic activity. A reaction intermediate, Na-H delta-center dot center dot center dot H delta'+-ONa, involving dihydrogen bonding between the negatively charged hydrogen atom bonded to sodium and the positively charged hydrogen atom bonded to oxygen is proposed for the reaction mechanism. (c) 2005 Elsevier B.V. All rights reserved.