The present invention provides a method of forming a wood-polymer composite structure and additive systems for use therein. The method of the invention includes extruding a heated mixture that includes from about 20 wt % to about 80 wt % by weight of a thermoplastic polymer, from about 20 wt % to about 80 wt % by weight of a cellulosic filler material, and from about 0.1 wt % to about 10 wt % by weight of an additive system. The additive system according the invention includes a 12-hydroxystearic acid salt or amide.