We have investigated the storage capability of microporous carbon materials for gaseous hydrogen both theoretically and experimentally. In the grand canonical Monte Carlo calculation the hydrogen molecules are physisorbed by van der Waals interactions with the surface atoms of carbon slitpores and carbon nanotubes. At room temperature the optimum pore geometry is a slitpore consisting of two graphite platelets separated by a distance that corresponds approximately to two times the diameter of a hydrogen molecule. In this case for a storage pressure of 10 MPa a maximum adsorbed hydrogen density of 14 kg/m(3) can be reached, which corresponds to a gravimetric storage capacity of 1.3 wt %. Only for low gas pressure a cylindrical geometry like that in carbon nanotubes can exceed the storage density of carbon slitpores owing to capillary forces.