The tangent-mass method is introduced to measure and calculate hydrogen storage capacity of carbon nanotubes by using a high-pressure microbalance. This method can effectively calibrate the effect of buoyancy and conveniently determine the adsorption isotherm. Single-walled carbon nanotubes were measured by this method to provide reference data for future studies. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.