The thermal properties of poly(ether sulphone) (PES) have been measured as a function of physical aging. The development of enthalpic relaxation with time, at several temperatures up to 70 K below T-g, has been investigated using differential scanning calorimetry (DSC). The kinetics of the aging process were determined from the change in enthalpy of the glass with time. The T-g was determined for glasses prepared at different cooling rates from 0.32 to 160 K min(-1) and the activation energy for glass formation was found to be 700 +/- 100 kJ mol(-1). It was compared with those determined by dielectric thermal analysis (DETA), dynamic mechanical thermal analysis (DMTA), and physical aging. A composite curve incorporating the measured T-g values by various techniques covering a relaxation time scale of 20 decades has been constructed. The heat capacity of PES has been measured over a temperature range, from which the change at T-g(Delta C-p (T-g)) was found to be 180 J kg(-1) K-1. Dielectric thermal analysis studies have also been carried out on aged specimens.