In this work we report a theoretical study on the atomic and molecular hydrogen adsorption onto Pd-decorated graphene monolayer and carbon nanotubes by a semi-empirical tight-binding method. We first investigated the preferential adsorption geometry, considering different adsorption sites on the carbon surface, and then studied the evolution of the chemical bonding by evaluation of the overlap population (OP) and crystal orbital overlap population (COOP). Our results show that strong C-Pd and H-Pd bonds are formed during atomic hydrogen adsorption, with an important role in the bonding of C 2p(z) and Pd 5s, 5p(z) and 4d(z)(2) orbitals. The hydrogen storage mechanism in Pd-doped carbon-based materials seems to involve the dissociation of H(2) molecule on the decoration points and the bonding between resultant atomic hydrogen and the carbon surface. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.