The effects of composition and interface modification on the tensile properties and the impact resistance of different woodflour/polypropylene composites (WPC) were studied. The woodflour was treated with vinyltrimethoxy silane as a coupling agent, and its effect on the WPC properties was examined. In addition, the tensile data were fitted with several semiempirical models to clarify the mechanical behavior of the composites. The Pukanszky parameters obtained from the fit of the tensile strength data revealed that the distribution of the filler was more homogeneous in the composites containing silane and that interaction between components was improved. The impact strength was maximal for the composites containing 30% of silane-treated woodflour. Morphological analysis by SEM revealed that the woodflour tended to form agglomerates when present in relatively high amounts in the formulations. For the silane-treated composites, the dispersion of the filler into the polypropylene (PP) matrix improved, although surface treatment blocked the inclusion of PP into the wood cells. This probably decreased the elastic moduli, even when the filler-matrix interaction was improved. The beneficial effects of the coupling agent on the WPC properties were greater in the materials containing a relatively low amount of filler (up to 30%); the effectiveness of 0.5% and 1% organosilane were very similar, demonstrating the effectiveness of the coupling agent. POLYM. ENG. SCI., 49:324-332, 2009. (C) 2008 Society of Plastics Engineers