Random copolymers of styrene with ortho- or para-bromostyrene differ substantially in their blend behaviour with partly sulfonylated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO). Copolymers of styrene with ortho-bromostyrene exhibit a very narrow window of miscibility in comparison with styrene-para-bromostyrene copolymer blends with SPPOs. The experimental results can be accounted for on the basis of individual segmental interaction parameters by applying mean-field theory.