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HTX, a poly(aryletherketone-co-sulfone), is characterized by size exclusion chromatography (SEC) at room temperature. As HTX is not soluble in any solvent compatible with the SEC instrumentation, a derivatization procedure is applied in order to prepare a modified copolymer with improved solubility properties. This derivatization consists in a sulfonation by concentrated sulfuric acid. The derivative polymer is soluble in N-methyl-2-pyrrolidone (NMP), a classical SEC solvent. Sulfonated HTX (HTXS) is a polyelectrolyte. Consequently, a salt is added to NMP to perform reliable viscometric and SEC analyses. The SEC method is standardized with the establishment of the universal calibration with poly(methyl methacrylate) standards and the determination of the Mark-Houwink-Sakurada relationship of HTXS in NMP-salt at 25-degrees-C. Finally, the reliability of the derivatization procedure and of the SEC method is confirmed by comparison with molecular masses from NMR.