Semicrystalline polymers have a metastable structure. During heating, reorganization processes can occur at temperatures between the glass transition and the final melting. When DSC measurements are performed at conventional heating rates, reorientation can occur. This can be the reason why the measured melting peaks are not representative of the crystalline structure of the original material. At fast heating rates, the actual heating time is so short that practical no reorientation occurs during the measurement. A commercially available heat flux DSC (METTLER TOLEDO DSC822(e)) was used to investigate the influence of heating rate on reorientation at heating rates between 1 K/min and 400 K/min. The measurements were performed on amorphous and sernicrystalline poly(ethylene terephthalate), PET. The heating rate dependence of the apparent glass transition temperature is discussed. For amorphous material, a criterion is given to estimate the minimum heating rate at which reorientation no longer occurs. Direct measurement of the melting of the crystallites present in the original starting material was possible at heating rates above 300 K/min. The original degree of crystallinity was approximately 1%. In the case of semicrystalline PET, the minimum heating rate at which recrystallization processes become less important, was determined using an Illers plot. (C) 2007 Elsevier B.V. All rights reserved.