The effects of HCl, HNO3, H2SO4 and HF acids on the purification and the electrochemical hydrogen storage of multi-walled carbon nanotubes (MWCNTs) were studied. The MWCNTs were synthesized on Fe-Ni catalyst by thermal chemical vapor deposition method. The X-ray diffraction and thermal gravimetric analysis results indicated that the MWCNTs purified by HF acid had the highest impurities as compared with the other acids. The N-2 adsorption results at 77 K indicated that all the samples were mainly mesoporous and the purified MWCNTs by HF acid had the highest surface area as compared with the other acids. The hydrogen storage capacities of the purified MWCNTs by the following acids were in ascending order as: H2SO4, HCI, HNO3 and HE It was found that the 1-2 nm micropores in the MWCNTs are very important for hydrogen storage. Further, the presences of catalyst and defective sites in MWCNTs influence the hydrogen storage capacity. (C) 2008 Elsevier B.V. All rights reserved.