Styrene/acrylonitrile (SAN) resin compositions are provided having both improved room temperature impact properties as well as good weatherability. The compositions comprise a mixture of SAN resin, an elastomeric halogenated copolymer comprising a C.sub.2 to C.sub.12 monoolefin, e.g. isobutylene, and a ring-substituted alklstyrene, e.g., para-methylstyrene, and a compatibilizing agent such as zinc stearate.