The blend miscibility of poly(N-methylmaleimide-alt-isobutene) [poly-(MeMI-IB)] with poly(acrylonitrile-co-styrene) (SAN) was investigated by means of measurement of the glass transition temperature of the blends. Poly(MeMI-IB) was found to be miscible with SAN of a specific range of acrylonitrile (AN) contents in the copolymer to produce transparent moldings. The refractive index changed from 1.58 to 1.53 and the dispersion decreased with increasing the amount of poly(MeMI-IB) in the blends. The stress optical coefficient of poly(MeMI-IB) was found to be reduced by the blending of SAN. The glass transition temperature, flexural modulus, and surface hardness of the blends increased with an increase in the amount of poly( MeMI-IB) in the blend.