This study examined the oxygen-transport properties of poly(ethylene terephthalate-co-bibenzoate) (PETBB55) crystallized from the melt (melt crystallization) or quenched to glass and subsequently isothermally crystallized by heating above the glass-transition temperature (cold crystallization). The gauche-trans conformation of the glycol linkage was determined by infrared analysis, and the crystalline morphology was examined by atomic force microscopy. Oxygen solubility decreased linearly with volume fraction crystallinity. For melt-crystallized PETBB55, extrapolation to zero solubility corresponded to an impermeable crystal with 100% trans glycol conformations, a density of 1.396 g cm(-3), and a heat of melting of 83 J g(-1). From the melt, PETBB55 crystallized as space-filling spherulites with loosely organized lamellae and pronounced secondary crystallization. The morphological observations provided a structural model for permeability consisting of impermeable platelets randomly dispersed in a permeable matrix. In contrast, cold-crystallized PETBB55 retained the granular texture of the quenched polymer despite the high level of crystallinity, as measured by the density and heat of melting. Oxygen solubility decreased linearly with volume fraction crystallinity, but zero solubility corresponded to an impermeable defective crystal with a trans fraction of 0.83 and a density of 1.381 g cm(-3).