The relation between the mechanical properties and the microstructure of PET has been investigated, combining results from WAXS, SAXS, FTIR, DSC, and uniaxial compression tests. The rigid amorphous fraction in the PET was explicitly taken into consideration in interpreting structure-property relations. WAXS results prove that glass crystallized PET with a high volume fraction of rigid amorphous material and small crystal size, on uniaxial compression shows a considerable loss in crystalline fraction. FTIR results in combination with these WAXS results suggest that during this loss in crystallinity, short-range conformational order is retained, while long-range structural order is lost. At the same time, material with small crystals and a high amount of rigid amorphous material was found to show unexpectedly low yield stress. It is concluded that in the interpretation of these phenomena it is necessary to take the three-phase structure of PET, including the rigid amorphous fraction into account. This is expected to hold for other semicrystalline polymers, where a rigid amorphous fraction is prominent, such as PHB, PBT, PEN, PEEK, etc. (C) 2004 Wiley Periodicals, Inc.