AbstractWith three polymers widely different in polarity—polyethylene terephthalate, polyethylene, and nylon 610—the water vapor permeability P of unoriented films has been shown to increase as the amorphous fraction Xa is increased, following the relation P = PaXa2. For polyethylenes, adherence to this relation is exhibited only with structures having densities above 0.94, a range in which the number of short chain branches is low. Films with densities lower than 0.94 exhibit higher film permeabilities than those calculated on the basis of the permeability of the amorphous areas Pa of “linear” polyethylene; this results from the increase in the number of short chain branches and hence, an increase in Pa of the lower density polyethylenes. The above variation of water vapor permeability with the square of the amorphous fraction may be theoretically accounted for by a linear variation of the diffusion and solubility coefficients with the amorphous fraction. With polyethylene terephthalate and nylon 610, the solubility coefficients do exhibit this linear relation. The square permeability relation appears to be a limiting case for structures in which crystallization does not impose restraints on the segmental motions important in permeability. Where restraints do occur, as in oriented films, a higher order dependency on the amorphous fraction exists. This was exemplified with biaxially and uniaxially oriented polyethylene terephthalate films, with which it was shown that orientation in oriented crystalline films effects a decrease in permeability in addition to that ascribable to crystallinity. This is not true for such films as polystyrene or stretched rubber which has not crystallized. Only those oriented structures in which the orientation is “locked‐in,” as by crystallization, exhibit a decrease in water vapor permeability. The relations of permeability and solubility coefficients to crystallinity have an important utility in that the extent of crystallinity or crystal density may be estimated with the proper data. On the basis of literature water vapor permeability data of Szwarc et al. for two high density polyethylenes at three different vapor pressure differentials, the crystal density of polyethylene was estimated to be 0.995, 1.008, and 0.998 or an average of 1.00, in agreement with Bunn's value calculated on the basis of x‐ray diffraction studies.