Diffusion of an array of molecules in micrometer‐thick films of plasma‐polymerized tetrafluoroethylene has been measured using an optical interferometric technique. The diffusivity is approximately independent of molecular size up to a molar volume of about 100 cm3and drops rapidly for larger molecules. For much larger molecules no penetration of the films is observed. These results suggest that plasma‐polymerized tetrafluoroethylene films are heavily cross linked and that this limits the size of the molecules that can penetrate the polymer. The temperature dependence and the molecular size dependence of the diffusivities are discussed in the context of free‐volume theory.