The synergistic effect of metallic Ti and Zr as co-dopants on the reversible hydrogen storage properties of NaAlH4 was investigated systematically. Metallic Ti and Zr powders were used directly and separately as dopants, as well as used as co-dopants together in the preparation of NaAlH4 by hydrogenation of ball-milled mixtures of NaH/Al. The hydriding/dehydriding properties of the composites were then investigated. It was found that the addition of Ti and Zr powders together as co-dopants on hydriding/dehydriding properties is superior to doping with Ti or Zr alone. The highest reversible hydrogen capacity of the hydride doped with Ti and Zr together as co-dopants is 4.34 wt% at 160 degrees C. The hydriding rate increases with the hydriding pressure increasing from 7.5 to 11.5 MPa. The dehydriding kinetics is improved with the dehydriding temperature increasing from 90 to 130 degrees C, and the dehydriding rate of the composite doped with Ti and Zr as co-dopants is 1.6 and 2.0 times that of the sample doped with Ti and Zr alone at 130 degrees C. Microstructure analysis reveals that the improvement of the hydriding/dehydriding properties of NaH/Al (NaAlH4) can be partially ascribed to the in situ interaction of active titanium-hydride and zirconium-hydrides formed in the ball-milling process and subsequently acted as the catalytic active sites on the surface of hydride matrix. The effect of lattice expansion on enthalpy change indicates that the further improved dehydriding property of the composite can also be attributed to a favorable thermodynamic modification of the bulk composite hydride co-doped with Ti-Zr powders. In a word, the synergistic effect on the improvement of hydriding/dehydriding properties by introducing the addition of Ti-Zr together as co-dopants can be ascribed to both the "superficial catalytic process" and the "favorable thermodynamic modification" of the composite. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.