Hydrogen adsorption on a single Si and SiO2 molecule, doped within C(6,6) and C(10,0) carbon nanotubes (CNTs), is studied using first-principles calculations based on density-functional theory. Two orientations of the H-2 molecule, inside the nanotubes, are compared. Our calculations revealed a rather weak hydrogen binding energy inside both types of pristine CNTs - namely, -0.51 eV/H-2 and -0.38 eV/H-2 for C(6,6) and C(10,0) nanotubes, respectively. When a single Si atom is doped in the interior surface of either type of CNTs, it tends to decouple from the wall and to drift towards the nanotube's axis. A Si atom can bind two hydrogen atoms more strongly (-1.4 eV/H-2 and -1.13 eV/H-2 on Si within metallic and semiconducting CNTs, respectively) than just a pristine CNT would do. A SiO2 molecule binds the hydrogen atoms even stronger, along with formation of water molecule within the metallic CNT. The corresponding binding energy of -1.73 eV/H-2 for the C(6,6) is found to be the highest one among the configurations considered. Based on our resuls, we believe that intrinsically Si and SiO2-doped CNTs can be considered as plausible candidates for enhancing the hydrogen adsorption properties. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.