Miscibility and phase behaviour in copolymer blends of partially phenylsulphonylated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) with poly(o-Auorostyrene-co-p(o)-chlorostyrene) have been studied by differential scanning calorimetry at temperatures up to 573 K. It was found that miscibility depends on the copolymer compositions and the degree of sulphonylation of the SPPO copolymers and on temperature. The width of the miscibility window in the blends containing copolymers with various p-chlorostyrene (pClSt) content and SPPO is much wider than in the corresponding blends containing copolymers with o-chlorostyrene (oCISt) and SPPO. It was also found that sulphonylated segments of SPPO copolymers enhanced miscibility in comparison with blends with poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). A qualitative account of these results was obtained by using the segmental interaction parameters according to the mean-field theory of phase behaviour.