To improve kinetics for hydrogen absorption of Mg and hydrogen desorption of MgH2, ternary composites were prepared from MgH2, Ni and Si or Ni2Si by a mechanochemical technique. The X-ray diffraction spectra of the resultant ternary composites after the initial hydrogen desorption treatment at 400 A degrees C in vacuum suggested that in all composites the nanocrystalline Mg2Ni would be formed in the intergrain region between Mg and Ni components, while in the MgH2-Ni-Si composite the nanocrystalline Mg2Si would also be formed in the intergrain region between Mg and Si components. However, Ni2Si did not form any alloys with Mg. The hydrogen absorption rate at 250 A degrees C for the MgH2-Ni-Ni2Si composite was comparable to that for the MgH2-Ni and MgH2-Ni-Si composites, while the hydrogen desorption rate at 250 A degrees C decreased in the order of MgH2-Ni > MgH2-Ni-Ni2Si > MgH2-Ni-Si. In contrast, the hydrogen desorption rate at 220 A degrees C for the MgH2-Ni-Ni2Si composite was faster than that for the MgH2-Ni composite, suggesting that Ni2Si was a key material in the improvement of hydrogen desorbability at lower temperatures. Moreover, the most plausible reaction model and the rate-determining process for hydrogen absorption and desorption at 250 A degrees C were determined.