The miscibility of random copolymers of o-chlorostyrene and p -chlorostyrene [P (oClSt-co-pCISt)] with partially phenylsulfonylated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) copolymers has been studied, using differential scanning calorimetry (DSC) to establish T(g) behavior. It already has been established that the isomeric effect of the chlorine substitution on miscibility is large. Thus the para-chloro-substituted styrenic homopolymer is miscible with all SPPOs containing more than approximately 5 mol % phenylsulfonylation, whereas the ortho-chloro-substituted homopolymer is immiscible with the entire range of SPPO copolymer compositions (and also with the respective homopolymers). As a result of this asymmetric behavior of the homopolymers, the width of the window of miscibility in blends now investigated containing copolymers with high pCISt content and SPPO is much greater than in the corresponding blends containing copolymers with large mole fraction of oClSt. These differences are reflected in the corresponding chi parameters calculated from analysis of the data. It was also found that the miscibility is temperature dependent and that the regime in the copolymer-copolymer composition plane shrank as the equilibrium temperature increased, results indicative of LCST behavior.