The small but significant difference in permeation rates of gaseous isotopes through dense polymers has been previously reported in several studies. Most results remain, however, restricted to hydrogen isotopes. In this study, the steady-state permeation selectivity of a series of seven different gaseous isotopes pairs (D/H, C-13/C-12, N-15/N-14, O-18/O-16,O-17/O-16, and Ar-36/Ar-40), covering a broad range of molecular weights (H-2, N-2, Ar, O-2, and CO2), has been investigated through two dense polymeric structures (PDMS and PTFE). It is shown that, with the notable exception of the H-2/HD pair, isotope permeation selectivity can be described for each polymer type through a single expression as a function of the relative molecular weight difference. We show, for the first time, that this behavior is consistent with Frisch and Rogers (1964) model. More interestingly, it suggests that the peculiarities of the H-2/D-2 pair, such as the high-relative mass difference or small size, may explain the inadequacy of the gas in a box quantum approach for this case. (c) 2008 Elsevier B.V. All rights reserved.