The He-reference method and modified tangent-mass method are introduced to measure and calculate the hydrogen storage capacity of carbon nanotubes by using high-pressure microbalance. These two methods can correctly calibrate the effect of buoyancy and determine the adsorption isotherm. Well-aligned carbon nanotubes were measured by two methods to provide reference data for future studies. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.