Permeability, diffusion and solubility coefficients are reported for biaxially orientated polyester films based on poly(ethylene terephthalate) [PET], poly(ethylene naphthalate) [PEN] and copolymers containing PET and PEN moieties. Data for cast amorphous sheets and materials produced with different biaxial draw ratios are compared. The crystallinity of the samples was assessed using differential scanning calorimetry and density measurements. The changes in the void structure at a molecular level were investigated using positron annihilation lifetime spectroscopy (PALS). The variation of the gas diffusion behaviour with the gas used (carbon dioxide, nitrogen, argon, helium and oxygen) reflects the effects of change in morphology on the solubility and diffusivity components of the permeability. The diffusivity of the gas is influenced not only by both the changes in the void size and content at a molecular level, but also by the effects of crystallinity on the percolation behaviour of the gas through the matrix. Changes in the extent of chain alignment also have a profound affect on the solubility of the gas in the matrix. The observed behaviour for the gas permeation can be interpreted as being the result of the complex interplay of changes in the crystalline content, the polymer chain alignment and the void structure of the amorphous phase. (C) 2000 Elsevier Science Ltd. All rights reserved.