Purified single-walled carbon nanotubes (SWNTs) with a large mean diameter of similar to1.85 nm, synthesized by a semicontinuous hydrogen arc discharge method, were employed for electrochemical hydrogen adsorption experiments. A maximal discharge capacity of 316 mAh/g, corresponding to similar to1.2 wt % hydrogen storage, for the purified SWNTs after high-temperature heat-treatment, was achieved reproducibly at 298 K under normal atmosphere. After 100 charge/discharge cycles, more than 81% of the maximal capacity was maintained. Moreover, the plateau of the discharge potential was observed over -0.6 V (vs. Hg/HgO) at a charge/discharge current density of 400 mA/g. Pretreatments, especially heat-treatment, had significant effects on the improvement in the electrochemical capacity of SWNTs. These results imply that the SWNTs have potential for application as electrode materials in secondary hydrogen batteries. (C) 2002 The Electrochemical Society.