A new method for poly(ether ether ketone) (PEEK) molecular-mass characterization by room-temperature (r.t.) size (steric) exclusion chromatography (s.e.c.) based on a derivatization procedure that improves the polymer solubility is described. Upon dissolution in 99.5% sulfuric acid at room temperature, PEEK is chemically modified by sulfonation of phenyl rings without polymer degradation. As complete sulfonation is achieved, the reproducibility of the chemical modification is ensured as well as the increase of molecular mass. Sulfonated PEEK is soluble at room temperature in different solvents suitable for s.e.c. N-Methyl-2-pyrrolidine (NMP) is selected as PEEK s.e.c. solvent. Owing to the polyelectrolyte nature of sulfonated PEEK, a salt (0.1 M LiBr) is added to NMP. Various broad-dispersity PEEK standards are used to establish specific and universal calibrations. The viscosity laws of sulfonated PEEK in NMP + LiBr (0.1 M), in methanesulfonic acid (MSA) and in 99.5% H2SO4 are determined. As sulfonated PEEK is a hygroscopic polymer, some spectroscopic methods are developed to determine the absolute concentration of the standards in order to perform accurate viscometric experiments giving the Mark-Houwink-Sakurada parameters in NMP-salt and in MSA. The s.e.c. universal calibration curve is constructed with poly(methyl methacrylate) narrow-dispersity standards.