Using grand canonical Monte-Carlo simulations, the adsorption capacities and isosteric heats of hydrogen on activated carbons, graphite nanofibers, and bundles of carbon nanotubes are estimated for identical thermodynamic states. These computations allow a systematic, meaningful, and unbiased comparison of the adsorption properties of hydrogen in such porous materials. The comparison shows that the hydrogen storage capacity can be optimized, but only to a limited extent, in adjusting the material pore sizes and functionalizing a part of the adsorption sites. Therefore, at room temperature and lip to 70.0 MPa, for the three Models of carbonaceous adsorbents. the hydrogen maximal excess adsorption is of the order of 2% of the adsorbent mass. (c) 2009 American Institute of Chemical Engineers AIChE J, 55: 538-547, 2009