The hydrogen storage properties and microstructures of Ti-doped NaAlH4 complex hydrides prepared by hydrogenation of ball-milled NaH/Al mixture with x mol% Ti powder (x = 0, 4, 6, 10) were investigated. It is found that hydrogen as ball-milling atmosphere is better than argon. The reversible hydrogen storage properties improve with increasing Ti content. As the milling time (t) extends from I to 24h, the hydrogen desorption capacity increases first and then decreases, and reaches a maximum capacity of 4.25 wt% at t = 16. The catalytic mechanism for hydrogen storage behavior of Ti-doped NaAlH4 is attributed to the presence of active small TiH1.924 and TiAl particles, which are scattered on the surface of much larger NaAlH4 (NaH/Al) globelets, acting as the catalytic active sites for the complex compound and playing an important catalytic role in the hydriding-dehydriding process. (C) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.