Miscibility in blends of random copolymers of poly[styrene-co-ortho(para)chlorostyrene] with partially sulfonylated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) has been studied. Evidence from DSC measurements shows the window of miscibility is influenced by both, the chlorine atom position on the benzene ring and the degree of sulfonylation of SPPO. Copolymers containing para-chlorostyrene exhibit miscibility window with SPPO of much higher degree of sulfonylation than the corresponding copolymers of styrene with ortho-chlorostyrene. Good agreement was obtained between the miscibility-immiscibility boundaries obtained experimentally and by those calculated in terms of the individual segmental interaction parameters using a mean-field approach.