Thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC) have been used to study the effect of physical ageing in polyetherimide (PEI: ULTEM 1000™). Ageing temperatures (T_a) between 48 and 8°C below the glass transition temperature (T_g) measured by DSC at 10°Cmin^‐1 heating rate (T_g=218°C), and ageing times (t_a) between 0 and 8640min, were analysed. Calorimetric results show the presence of an endothermic peak, associated with the enthalpy recovery at the glass transition. This peak shifts towards higher temperatures and its height and area increase as the annealing time increases for a given T_a. The maximum enthalpy relaxation is achieved at T_a=T_g‐18°C (T_a=200°C) for every annealing time, indicating that this is the optimum temperature to age this material. TSDC results show for each T_a a shift of the α peak towards higher temperatures, and a considerable decrease in maximum current and area as t_a increases. The activation energies (E_a), calculated from the TSDC curves for the α relaxation, are mainly independent of the annealing temperature and time. The pre‐exponential factor for the relaxation time (τ₀), evaluated from the same curves, shows a slight decrease when the annealing time is increased at a given temperature. © 1998 SCI.