Abstract: The present invention provides a method of forming a polymer-wood composite structure and additive systems for use therein. The method of the invention includes extruding a heated mixture that includes from about 20% to about 80% by weight of a thermoplastic polymer, from about 20% to about 80% by weight of a cellulosic filler material, and from about 0.1% to about 10% by weight of an additive system. The additive system according the invention includes a blend of from about 10% to about 90% by weight of a nonionic compatibilizer having an HLB value of from about 9 to about 19 and from about 10% to about 90% by weight of a lubricant. Use of the method and additive system according to the invention facilitates the production of highly filled polymer-wood composite structures at a very high output rate while maintaining commercially acceptable surface appearance.