We have studied the kinetics of gas transport through films made of self-assembled cellulose nanofibrils (CNF) by a time-resolved mass spectroscopy technique. Few micrometers thick films deposited on polylactic acid (PLA) substrates act as impermeable barriers for CO₂, O₂, and N₂ and reduce the ²H₂ (deuterium) and He permeation flux by a factor of ~10³ with respect to the uncoated substrate. Penetrant transport is controlled by the solution-diffusion mechanism and the coating acts as a diffusive barrier. ²H₂ and He diffusivity values are in the 10^–10 and 10^–9 cm² 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.