Two thermal analysis techniques, differential scanning calorimetry (DSC) and thermostimulated currents, are used to characterize two Poly(ethersulfones) (PESs) obtained with the same process by two different manufacturers. The glass transition temperature observed by DSC is the same for both PESs and equal to 235 degrees C. When PES samples are heated above the glass transition temperature (250-260 degrees C), one initially white and opaque PES (B) looks transparent while the other one (A) stays in the initial stage. The PES B is visibly degraded upon increasing at a lower temperature than PES A. Below the glass transition temperature, two complex relaxation modes alpha and beta situated at +170-175 and -120 degrees C, respectively, are observed. The analysis of the fine structure shows that they are constituted of elementary processes characterized by relaxation times following compensation laws. For PES A at high temperatures the cooperative movements are precursors of the glass transition. For PES B a double compensation phenomena reveals the biphasic character of the vitreous phase. The comparison between the two PESs reveals different microstructures that could be due to different processing conditions. (C) 2000 John Wiley & Sons, Inc.