Miscibility in random copolymers of p-fluorostyrene-co-p(o)-chlorostyrene (poly(p-FSt-cop(o)-ClSt)) with partially sulphonylated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) has been studied by differential scanning calorimetry (DSC). It was found that miscibility predominantly depends on the degree of sulphonylation of SPPO copolymers. Copolymers of pFSt with pClSt exhibit miscibility with SPPO with a higher degree of sulphonylation than the copolymers containing oClSt. Upon annealing to temperatures of 523 and 573 K, only small changes in the miscibility regime were observed. Using the mean-field theory approach, the miscibility behaviour was analysed in terms of the individual segmental interaction parameters.