Hydrogen storage capacities of raw, oxidized, purified and Fe-doped multi-walled carbon nanotubes (MWCNTs) were studied by electrochemical method. Based on transmission electron microscopy and Raman spectroscopic data, thermal oxidation removed defective graphite shells at the outer walls of MWCNTs. The analysis results indicated that the acid treatment dissolved most of the catalysts and opened some tips of the MWCNTs. Thermal gravimetric analysis and differential scanning calorimetry results illustrated that by oxidation and purification of MWCNTs, the weight loss peak shifts toward a higher temperature. N-2 adsorption isotherms of the purified and oxidized MWCNTs showed an increase in N-2 adsorption below P/P-o = 0.05, suggesting that microporous structures exist in the purified and oxidized MWCNTs. The electrochemical results revealed that the Fe-doped MWCNTs produced the highest hydrogen storage capacities compared to the other samples in various sweep rates. According to electrochemical analyses, the peak currents of hydrogen adsorption/desorption increased by increasing the catalyst's active surface. (C) 2008 Elsevier B.V. All rights reserved.