In this study, dielectric relaxation spectroscopy experiments were carried out during heating in the alpha relaxation region of PET. The results for low heating rates were consistent with isothermal measurements. Moreover, the results expressed as a function of temperature were much clearer than those expressed as a function of frequency. However, the loss peaks obtained in these non-isothermal experiments showed a systematic shift on the temperature axis when the heating rate increased. A significant but nearly constant thermal lag was observed in experiments with heating at 1 K min(-1) when using different equipment. It appears that isothermal data obtained in a well calibrated apparatus can be used to adjust the temperature axis of dielectric results obtained in ramp temperature experiments.