In this paper, we present our recent results on the effect of moisture during prolonged cycling on hydrogen storage properties of nanostructured MgH2 hydride with V and Ti catalysts additions synthesized by ball milling. The hydrogen charge and discharge stability of the nanocomposite hydride has been tested at 300degreesC for up to 1000 cycles under hydrogen containing 101 ppm moisture. Between the first and the 500th or 1000th cycle, an increase of hydrogen storage capacity by about 5% is observed in dynamic and PCT measurements. This could be due to structural relaxations. While absorption kinetics remain fast, the results show a significant and systematic slow-down of the desorption rates by about a factor of two during cycling. The X-ray diffraction patterns of nanocomposite performed before and after 1000 cycles reveal that the peak shape for magnesium remains unchanged indicating that the crystal growth is negligible. This microstructural stability during cycling suggests that the decreasing desorption rate of the nanocrystalline magnesium-based composite is not induced by any internal structural modification. On the other hand, the presence of moisture in the hydrogen gas during cycling induces surface effects which most likely cause the decrease of the hydrogen discharge flow rate. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.