The effect of impurities on hydrogen storage characteristics of nanocomposite MgH2-V 5 at% was studied. The impurities considered were nitrogen, oxygen and carbon dioxide in concentration of 0.53%, 0.43% and 1.96%, respectively. Each experiment consisted in performing 10 absorption/desorption cycles in pure hydrogen, switch to impure hydrogen for 50 cycles (only 10 cycles in the case of CO2) and then back to pure hydrogen for 10 cycles to verify the recovery of the absorption characteristics. In the case of nitrogen contamination, 50 cycles resulted in a loss of hydrogen capacity of about 20% and a reduction of hydrogen sorption kinetics. The loss of capacity was not recovered after cycling under pure hydrogen but the hydrogen sorption kinetics reverted back to their original values. Cycling 50 times under oxygen contaminated hydrogen produced a permanent loss of hydrogen capacity of 45% but had no effect on sorption kinetics. For CO2 contaminant, only 10 cycles resulted in a total loss of capacity. However, 80% of the original hydrogen capacity could be recovered after 50 cycles under pure hydrogen. (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.