The effect of crystallization from the glassy state (cold crystallization) on the oxygen-barrier properties of copolyesters based on ethylene terephthalate with up to 10 mol % isophthalate, phthalate, or naphthalate was examined. Generally, crystallization affected diffusivity D more than solubility S; thus, the reduction in permeability P reflected primarily a reduction in D. Systematic changes in crystallinity made it possible to test free-volume concepts in which permeation of a small gas molecule through a semicrystalline polymer is viewed as proceeding through the amorphous regions with an increased pathway (tortuosity) imposed by plateletlike crystallites. Of the copolymers studied, those with the highest isophthalate or phthalate content (10 mol %) conformed to the simple two-phase model with constant densities of an impermeable crystalline phase and a permeable amorphous phase. Within the two-phase model, solubility S correlated linearly with the volume fraction of the amorphous phase, and diffusivity D depended on crystallinity in accordance with the Nielsen model for randomly dispersed platelets with an aspect ratio of 4. The reduction in permeability of the other examined copolyesters could not be described only by the filler effect of crystallites. Data on solubility demonstrated a decrease in amorphous-phase density upon cold crystallization (de-densification) like that previously reported for polyethylene terephthalate. Increasing the isophthalate or phthalate content reduced the dedensification effect, and 10 mol % of these comonomers was sufficient to eliminate the effect altogether. In contrast, 10 mol % naphthalate did not prevent de-densification. This was attributed to different effects of kinked and linear comonomers on chain packing in the amorphous phase.