The structural changes occurring when amorphous cold-drawn poly(ethylene terephthalate) films are annealed at different temperatures (50 degrees C-240 degrees C) for different annealing times (10-10(4) s) were investigated by means of X-ray diffraction and microhardness techniques. The X-ray results reveal the appearance of smectic order at 60 degrees C with a period of 10.7 Angstrom. At 70 degrees C, a layer structure in the scale of 110 Angstrom emerges. Finally, triclinic order is observed above 80 degrees C. The appearance of a layer structure prior to the development of triclinic crystals is associated with a density difference along the molecular direction produced by a molecular tilting mechanism. The microhardness behaviour of annealed cold-drawn PET films is correlated to the developing morphologies. At high annealing temperatures (>100 degrees C), the plastic component of hardness is shown to vary with the occurring microstructural changes. Results indicate that the hardness of the amorphous intrafibrilar regions is higher than that of a fully amorphous material. The indentation anisotropy, Delta H, which is related to the elastic recovery of the material shows a conspicuous decrease at T-a similar to 70 degrees C, which is explained in terms of a relaxation of the fibrils in the chain direction.