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We have studied the kinetics of gas transport through films made of self-assembled cellulose nanofibrils (CNF) by a timeresolved mass spectroscopy technique. Few micrometers thick films deposited on polylactic acid (PLA) substrates act as impermeable barriers for CO 2 , O 2 , and N 2 and reduce the 2 H 2 (deuterium) and He permeation flux by a factor of ∼10 3 with respect to the uncoated substrate. Penetrant transport is controlled by the solution-diffusion mechanism and the coating acts as a diffusive barrier. 2 H 2 and He diffusivity values are in the 10 −10 and 10 −9 cm 2 s −1 range, respectively, and their migration occurs by thermally activated process with 39 ± 1 and 33 ± 2 kJ mol −1 activation energy. Positron annihilation lifetime spectroscopy analysis indicates that the diffusive path between the packed nanofibrils consists of elongated cavities with cross-sectional size ∼ 0.31 nm. Results evidence that the selective transport of the small size penetrants is due to sieving effects and that small penetrant transport occurs in configurational diffusion regime.