A paracrystalline macrolattice model has been used to provide a detailed description of the morphological changes which occur during the annealing of an oriented polyester, poly(ethylene terephthalate) (PET) at temperatures between 100 and 200 degrees C. The model structures were obtained by fitting the observed and calculated SAXS patterns using an automated series of computational routines. Accurate pictures of the paracrystalline structures, which have been generated according to the precise values of the model parameters, were able to provide a quantitative insight into the growth of the crystallites during the annealing process. The limitations of using a two-phase model with cylindrical symmetry to describe the structure of PET film are discussed.