Molecular characterization of PES by light scattering, viscometry, and size exclusion chromatography (SEC) is described. A Mark-Houwink-Sakurada relationship in NMP + 0.1 M LiBr is established. From specific and universal calibrations, absolute molecular masses of PES are obtained by SEC. Number average molecular masses from SEC and H-1 NMR are compared. The study of PES conformation in dilute solution reveals a high flexibility of the PES chain in a good solvent. The sigma factor is close to 1, and the Kuhn statistical element contains approximately six repeat units.