Density functional theory (DFT) calculations have been employed for a systematic study of electronic dispersion and morphologic characteristics (length and angle of chemical bonds) of bare and hydrogenated narrow zigzag single walled carbon nanotubes (SWCNTs), d < 7 angstrom. For these narrow tubes, the "band gap" and the bond character show a strong dependence on the diameter of the tubes. Our results show that exo-hydrogenation changes the band gap substantially. The gap shows essential dependence on the coverage of the exo-hydrogenation and increases with decreasing tube diameter. These properties can be useful in sensing and characterising applications. However, endo-hydrogenation in wider tubes occurs mainly by storing H(2) gas molecules inside them and their electronic structure changes slightly whereas for very narrow tubes, there is an interplay between the curvature and chirality effects. The nature of hydrogen adsorption in these cases can be a mixture of the internal C H bonds and H(2) molecules and existence of free atomic hydrogen gas inside the tubes cannot be ruled out. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.