The invention relates to a methylmethacrylate-butadiene-styrene (MBS) core/shell polymer impact modifier containing functionalization in the shell. The functionalized MBS is useful as an impact modifier in engineering resins, and especially in blends of engineering resins, particularly where the blend contains both functional and non-functional resins. One specific engineering resin blend having excellent low temperature impact performance is a polycarbonate (PC)/polyethylene terephthalate (PET) blend with the functional MBS of the present invention.