The preparation and characterization of modified and unmodified polypropylene (PP)-wood powder (WP) composites were done under fixed processing conditions. Different techniques were used to study the effect of both WP size and content, as well as compatibilizer content, on the properties of the composites. The results point to the fact that, WP settles in the amorphous part of the matrix and creates new crystalline phases or zones. Scanning electron microscopy micrographs show a relatively even distribution of WP in the PP matrix, which contributes to improvements observed in the properties of the material. Hg-porosimetry results indicate that the PP matrix, which has a low pore volume, filled the pores in the WP particles. This reduced the total volume of pores in the PP-WP composites. This observation was also supported by a general decrease in gas permeability of the material. Thermal analysis results indicate that the presence of both WP and maleic anhydride grafted polypropylene (MAPP) leads to an increase in enthalpy (crystallinity) values, but to a decrease in lamellar thickness in the composites. The thermal stability of the composites improves somewhat compared to that of PP. There were distinctive differences between the results for composites containing different WP particle sizes, as well as for composites prepared in the presence and absence of MAPP. It is clear from the results that the presence of MAPP generally improves the tensile properties of the composites. Larger WP particles gave rise to better tensile properties, in the presence and absence of MAPP. (c) 2006 Wiley Periodicals, Inc.