A coupling agent formulation for cellulosic material-polymer composites is provided. The formulation includes a) at least one organic peroxide; and b) at least one of i) at least one zinc-containing reagent; and/or ii) at least one silicon-containing reagent. The coupling agent formulations produce polymer matrix cellulosic material composites with improved strength and aging characteristics. The improved strength may be related to physical properties such as improved stiffness, toughness or tensile strength. A masterbatch utilizing the coupling agent formulation is provided, as well as a method making the masterbatch.