The isotopic separation factors for T-2/H-2 and D-2/H-2 mixtures adsorbed in various organic frameworks have been calculated using path integral Monte Carlo simulations. The results are in fair agreement with the available experimental data, and show a very strong dependence on temperature. The selectivity shows a less pronounced dependence on the external pressure, but different behaviours are found for different materials. They are analyzed from the point of view of the solid-fluid potential energy surface. (c) 2008 Elsevier B. V. All rights reserved.