Oxygen transport was coupled with other methods to study the relationship of gas barrier to solid state structure in the PET blown bottle wall. Commercial 2-L carbonated soft drink bottles blown under different process conditions were studied. Crystallinity determinations from heat of melting, density, glycol trans fraction, and oxygen solubility were compared. The reasons for lack of correlation between conventional crystallinity methods based on heat of melting and density were elucidated, and neither was found to be reliable. An alternative approach to determining crystallinity based on oxygen solubility gave reliable results and revealed fundamental characteristics of the amorphous phase, such as amorphous phase density and amorphous phase oxygen solubility. Dedensification of the amorphous phase was responsible for higher oxygen permeability of bottle walls processed under conditions of higher thermal exposure (temperature/time) despite the parallel increase in volume fraction of impermeable crystals. The strong dependence of dedensification on process temperature was explained in terms of the temperature-volume relationship of the amorphous phase. A low process temperature minimized amorphous phase dedensification and delivered a bottle wall with the best oxygen barrier. (C) 2004 Wiley Periodicals, Inc.