Some poly(ether ether ketone) (PEEK) samples were analysed by light scattering (l.s.) before and after complete sulfonation. Afterwards, the average molecular masses and molecular-mass distribution of the same PEEK samples are determined by high-temperature size (steric) exclusion chromatography (s.e.c.) for unmodified PEEK and by room-temperature s.e.c. (r.t.s.e.c.) for totally sulfonated PEEK. From this work, it appears that l.s. and s.e.c. measurements using sulfonated or unsulfonated PEEK sample give identical results within the limit of the s.e.c. and l.s. accuracy. This verification assesses the reliability of the derivatization method used to perform r.t.s.e.c. It is also shown that number-average molecular masses ((M) over bar(n)) of fully fluoroarylketone-terminated PEEK determined by F-19 nuclear magnetic resonance experiments agree very well with (M) over bar(n) values from s.e.c. These results support the validity of both s.e.c. methods used in this work. Finally, a PEEK-carbon fibre composite is characterized. The fibre content is evaluated. The derivatization process is applied to the matrix and the molecular parameters are obtained by r.t.s.e.c.