From a detailed study of the syntheses and physical properties of semicrystalline poly[aryl ether(ketone-co-sulfone)] model compounds, this paper leads to a better understanding of the chemistry of the polymer synthesis and of its thermal behaviour. Long, crystallizable sequences are shown to arise from the heterogeneous nature of the synthesis. These sequences are responsible for the high melting points observed. Conclusions are drawn on the necessity for a revision of the synthesis procedure in order to avoid the appearance of such sequences.