Polypropylene-based composites reinforced with ground wood pulp (pulp stone) were prepared in the reinforcing range of 20-50 wt %. The mechanical properties of the system were evaluated through the observation of a stiffening effect of the prepared materials, which reached Young's moduli up to 2.3 times higher than that of plain polypropylene, for formulations with 50 wt % ground wood pulp as a reinforcement. In contrast to this result, the ultimate tensile strength decreased because of the different polarities of the components of the composite. To overcome this problem, maleic anhydride grafted polypropylene was added as a coupling agent (6 wt %, with respect to the ground wood pulp content, as the optimal composition), giving rise to an important improvement in the ultimate tensile strength close to 125%, in comparison with the uncoupled composite, for the composite filled with 50 wt % ground wood pulp. The addition of maleic anhydride grafted polypropylene also diminished the capacity for absorbing water, improving the durability of the composite under wet conditions. (c) 2007 Wiley Periodicals, Inc.