Density functional theory (B3LYP) calculations using 3-21G and 6-31G(d) basis sets were performed for four different lengths of (3,3) and (4,4) armchair single-walled carbon nanotubes to examine the effect of tube length on the chemisorption process. The significant changes in the CC bond lengths by H-chemisorption are due to the change of hybridization of carbon atom(s) from sp2 to sp3 at the chemisorption site(s). In general, 3-21G basis set overestimates the bond distances and reaction exothermicities compared to 6-31G(d) basis set; however, both the basis sets reveal similar trend. The exothermicity for the addition of two H atoms is about 2–2.5 times that of one H chemisorption except for H (1,3) addition. The present study reveals that the positional preference for the chemisorption of two hydrogen atoms is not same for the armchair and zigzag type nanotubes. The reaction energies for hydrogen chemisorption on the surface of (3,3) nanotubes are substantially higher than for (4,4) SWNTs, probably due to the greater curvature and larger strain in the former case. The change of tube length has significant effect on the reaction energies of hydrogen chemisorption and band gap values. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007