Miscibility of blends of poly(acrylonitrile-co-styrene) with poly left bracket styrene-co-(maleic anhydride) right bracket and poly left bracket styrene-co-(N-phenylmaleimide) right bracket was determined by measurement of their glass transition temperatures by dynamic mechanical testing. It was found that poly(acrylonitrile-co-styrenes) are miscible with poly left bracket styrene-co-(maleic anhydride)s right bracket and poly left bracket styrene-co-(N-phenylmaleimide)s right bracket within specific ranges of copolymer composition for each blend. The boundaries between domains of miscibility and immiscibility were expressed by two straight lines intersecting at the origin, where the abscissa and ordinate represent the compositions of the respective copolymers in volume fraction.