The miscibility and phase separation behaviour of random copolymer blends of poly [ortho(para)-fluorostyrene-co-ortho(para)bromostyrene] (poly[o(p)FSt-co-o(p)BrSt]) with partially sulphonylated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) copolymers has been studied by differential scanning calorimetry (DSC) at temperatures up to 573 K. It was found that these systems exhibited miscibility-immiscibility behaviour which was dependent on the halogen atom, its position on the benzene ring and predominantly on the degree of sulphonylation. Copolymers of oFSt and pFSt with pBrSt exhibited miscibility with SPPO with higher degree of sulphonylation than copolymers containing oBrSt. The miscibility behaviour was analysed on the basis of the mean-field theory in terms of the individual segmental interaction parameters.