The structural and electronic characteristics of fully hydrogenated armchair and zigzag carbon nanotubes have been determined by quantum chemical methods. With use of line group symmetries, the structures of nanotubes up to 10 nm in diameter could be optimized by periodic B3LYP calculations. "In-out" isomerism is shown to signiticantly stabilize perhydrogenated carbon nanotubcs, the energetically most favorable structures being those with 1/3-1/2 of the carbon atoms endo-hydrogenated. In favored nanotubes the ratio of endo- to exo-hydrogens is 1:1, the stabilities increasing as a function of the diameter of the nanotube. The calculated band gaps indicate that the perhydrogenated carbon nanotubes are insulators. (C) 2007 Elsevier B.V. All rights reserved.