We report measurements of gas permeation through ultrathin liquid layers. The gas flux penetrating nanometer thick free-standing smectic films is determined experimentally as a function of membrane thickness and temperature. We observe an unusual film thickness dependence of the permeation in very thin films. For its interpretation, we introduce a molecular model that considers sorption and diffusion of gas molecules in the liquid film as well as the dynamic exchange between the film and the external gas phase. From the fit of experimental data, we obtain the ratio of the solubility of gas in the smectic phase and the diffusion coefficient for gas molecules in the liquid and an energy barrier for gas molecules to enter the liquid film. The measured permeation is strongly temperature dependent; at the transition to low-temperature smectic modifications with in-plane molecular positional order, the films become almost impermeable.