We investigate the hydrogen absorption capacity of two tubular structures, namely, B Be -substituted single-wall carbon nanotube (SWNT) and Ti covered single-wall boron nitride nanotube (SWBNT) using first-principles plane wave method. The interaction of H2 molecules with the outer surface of bare SWBNT, which is normally very weak, can be significantly enhanced upon functionalization by Ti atoms. Each Ti atom adsorbed on SWBNT can bind up to four H2 molecules with an average binding energy suitable for room temperature storage. While the substitution process of Be atom on SWNT is endothermic, the substituted Be strengthens the interaction between tube surface and H2 to hold one H2 molecule. © 2007 The American Physical Society.