To optimize the performance of carbon nanotubes in the area of hydrogen storage we explore stable structures that can form upon hydrogenation. Through free energy calculations using first-principles DFT we find that at room temperature the trans-hydride structure, i.e., one in which H-atoms chemisorbed on nearest-neighbor sites alternate between outside and inside of the nanotube is the most stable structure under moderate hydrogen dosage for tubes with diameter > 1.2 nm. Such structures are energetically stable with respect to recombination into H-2 or decomposition into hydrocarbons like benzene or ethylene. We also find that the IR spectrum of the trans-hydrides display striking dependence on nanotube chirality. (C) 2011 Elsevier B. V. All rights reserved.