Characterization of high-temperature poly(ether sulfone) (HTPES) molar mass was performed using light scattering, viscometry and size-exclusion chromatography (SEC). In pure Nmethyl-2-pyrrolidone (NMP), a large increase of the reduced viscosity at low concentration was observed. By addition of a salt such LiBr, this effect was suppressed. Using the viscosity laws of HTPES and poly(methy1 methacrylate) (PMMA) in NMP + LiBr the SEC method was standardized via the universal calibration established with PMMA standards. The Mark-Houwink-Sakurada (MHK) relationship for HTPES in NMP containing 0,1 mol/L LiBr at 25 "C with the intrinsic viscosity [q] and molecular weight M afforded the MHK constants K = 33,8 . lo-' dL . mol" . g"" and a = 0,70. The weight-average molar masses obtained from SEC were very close to those obtained from light scattering. A comparison of number-average molar masses obtained from 'H NMR, potentiometric titration of chain-ends, and from SEC has shown more discrepancy for high molar masses. The study of dilute solution properties of HTPES macromolecules has shown the marked decrease of anisotropy with the number of repeating units, the low values of the characteristic ratio and of the Kuhn segment. All these conformational properties are similar to those of freely rotating chains. a) Research Associate of the National Fund for Scientific Research (FNRS).