Hydrogen storage in single-walled carbon nanotubes (SWCNTs) was investigated by using a dual quartz-crystal microbalance (QCM). The chamber pressure can be adjusted from 10(-9) torr up to 1 MPa. The dual QCM decreases the frequency fluctuations that originate from environmental variables such as temperature and pressure. Our moisture-free chamber enables us to measure accurately the hydrogen storage capacity in carbon nanotubes. Highly purified SWCNTs store about 0.2 wt% of hydrogen at room temperature under 0.5 MPa through physical adsorption. Our SWCNTs without baking adsorb up to 1 wt% of water, often obscuring the hydrogen storage capacity. Our results suggest that the hydrogen storage capacity relies strongly on the functional groups remaining on the SWCNTs which may enhance the storage capacity. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.