The voiding behaviour of a filled polyester film, from polyethylene terephthalate as a function of drawing, was followed by measuring density and optical density and characterized using differential scanning calorimetry and wide-angle X-ray scattering. The data obtained were analyzed to follow the development of voids, molecular orientation and crystallinity. The results demonstrated the importance of the viscoelastic nature of a polymer to void formation. It was shown that the temperature and the draw rate are interchangeable parameters in terms of their effect on the development of a void. It was also suggested that finding the correct draw temperature of the first draw stage of a biaxial film process is the most important factor when a highly voided polyester film is desired. © 2005 Elsevier B.V. All rights reserved.