In observation of the state of hydrogen molecules within the carbon nanopore, the excess adsorption amounts of hydrogen on the multi-walled carbon nanotubes (MWCNTs) were measured at equilibrium pressure-temperatures from 0.1 to 12.3 MPa and 123 to 310 K. The principles of thermodynamic equilibrium and a higher order Virial adsorption coefficient were applied to determining the maximum surface coverage of hydrogen molecules on the adsorbent surface. The thermodynamic equilibrium-based adsorption model was linearized to estimate the interaction energy among the adsorbed hydrogen molecules at each adsorption equilibrium state. The results demonstrate that the interaction energies among adsorbed hydrogen molecules are positive in the lower temperature region (<200 K) and reach the maximum value around a temperature from 160 to 180 K. However, it will gradually be negative when the temperature is approaching 230 K. In other words, the confined hydrogen molecules repulse each other in the low-temperature environment while they attract each other at the ambient temperature. It implies that the dissociativeness of hydrogen occurred in the experimental pressure-temperature range, and it is also suggested that the temperature between 160 and 180 K could be a preferable condition to make full use of physical and chemical adsorption of hydrogen molecules on the adsorbent. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.