The hydrogen adsorption simulations were carried for several model AIPOs (VPI-S, AIPO-5, AIPO-11 and AIPO-25) employing the Grand Canonical Monte Carlo (GCMC) simulations at 77 K to investigate the effect of pore size and the pore volume on the hydrogen uptake. The adsorption capacity showed no relationship with the pore size, surface area and micropore volume of AIPOs. However, the adsorption capacity per unit micropore volume increased as the pore size decreases. The heat of adsorption also increased as the pore size decreases. For all model AIPOs, the hydrogen exists homogeneously near the oxygen atoms in the framework. (c) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.