The electrochemical behavior and the reversible hydrogen storage capacity of multiwalled carbon nanotubes (MWNTs) have been investigated both in alkaline (6 M KOH) and acid (0.3 M H2SO4) electrolytic solutions by cyclic voltammetry and constant current charge/discharge measurements. Different purification treatments were performed on as-prepared MWNTs to improve hydrogen uptake. About 0.1 wt % of hydrogen was reversibly stored in MWNTs after nitric acid treatment. Vacuum annealing (950degreesC, 1 h) of MWNTs lowered the electrochemical hydrogen uptake. Finally, electrochemical oxidation of MWNTs at high potentials allowed an increase in hydrogen charge capacity to 0.3 wt %. (C) 2004 The Electrochemical Society.