We present diffusion Monte Carlo calculations of D 2 adsorbed inside a narrow carbon nanotube. The one-dimensional D 2 equation of state is reported, and the one-dimensional character of the adsorbed D 2 is analyzed. The isotopic dependence of the constitutive properties of the quantum fluid are studied by comparing D 2 and H 2. Quantum effects due to their different masses are observed both in the energetic and the structural properties. The influence of the interatomic potential in one-dimensional systems is also studied by comparing the properties of D 2 and 4 He which have nearly the same mass but a sizeably different potential. The physics of molecular hydrogen adsorbed in the interstitial channels of a bundle of nanotubes is analyzed by means of both a diffusion Monte Carlo calculation and an approximate mean-field method.