The permeability of a number of thin organic films toward oxygen and nitrogen has been measured. For an ethyl cellulose film the studies were extended to include CO2, A, He, and H2. The enrichment of a binary gas mixture in a single stage of permeation has been calculated for the extreme cases of perfect mixing and no mixing. Application of these results indicates that the use of a fractional permeation process may be of practical importance in effecting the separation of oxygen from air, helium from natural gas, and hydrogen from coke‐oven gas, as examples.