Small angle neutron scattering (SANS), scanning transmission electron microscopy (STEM) and gas adsorption, were applied to study the morphology and hydrogen adsorption prop-erties of a wood-based ultramicroporous carbon (UMC) and a poly(furfuryl alcohol) derived carbon (PFAC). The polydispersed spherical model and the Guinier analysis of the scattering profiles were applied to obtain morphological parameters such as average pore size and pore size distribution of the two carbons; the results agreed reasonably well with indepen-dent gas sorption measurements and structural analysis by electron microscopy. The den-sity of hydrogen physisorbed in these two carbons at room temperature and at moderate pressures was investigated by in situ SANS measurements. The experimental data, ana-lyzed using a modified Kalliat model for decoupling scattering contributions from pores of different sizes, indicate that the molecular hydrogen acquires high densities preferen-tially in pores of subnanometer size at all measured pressures. These results support exist-ing quantum mechanical and thermodynamical models that have predicted that the narrowest pores enhance the adsorption due to the overlapping of the potential fields from both wall sides in the slit-like pores. The structural information at a nanometer level gained via this work could guide the new development of porous-carbon based materials for hydrogen storage.