The analysis of the process of gas and vapour permeation through dense polymers is of primary importance for packaging, controlled release and membrane separation processes. A significant amount of polymer permeability data has already been reported, especially for permanent gases, such as N 2 , O 2 , H 2 , CH 4 and CO 2 , although a very small amount of data is available for ammonia (NH 3 ). In this case, the experimental results show the faster permeation of NH 3 in comparison to CO 2 , which is in agreement with the solution diffusion model predictions. NH 3 is smaller and more condensable than CO 2 . In this study, the solubility, diffusion coefficient and permeability of NH 3 , CO 2 and N 2 in ten different polymers were investigated between 5 and 50°C. A reverse NH 3 /CO 2 permeation selectivity is occasionally observed for dense fluorinated polymers (PTFE, FEP, Hyflon AD and Teflon AF). This unusual behaviour is interpreted in the light of the diffusion and sorption coefficients obtained from time lag experiments which tend surprisingly to show in fluorinated polymers: a NH 3 /CO 2 solubility selectivity systematically lower than expected by the usual correlations and, in some cases, lower diffusion coefficients for NH 3 than for CO 2 . This peculiar result is interpreted from the differences between NH 3 and CO 2 interactions with fluorine atoms, similarly to the differential solubility phenomena of these two species which have previously been clearly established in fluorinated liquids.