Using ab-initio density functional theory, we investigate both molecular and atomic hydrogen adsorption properties for sp 2-bonded carbon materials, such as graphene, fullerenes, and nanotubes, and their modified structures. Weak molecular hydrogen binding depends on different binding sites and hydrogen orientations. The energy barrier of the drift motion of a hydrogen molecule on the surface of graphitic materials is estimated to be small. We find that some modifications of the sp 2bonding characters of graphitic materials, resulting from structural deformation or chemical doping, tend to enhance molecular hydrogen binding, and thus may increase the desorption temperature. Atomic hydrogen binds chemically or relatively strongly to the host carbon materials, resulting in a local modification of sp 2 carbon to sp 3 carbon. However, such strong atomic bonds easily breaks in the presence of unbound atomic hydrogen to form more favorable molecular hydrogen.