The main objective of this study was to investigate the effect of different additives, namely, maleic anhydride, alumina tribydrate (ATH), and a mixture of both on wood flour/polyolefin (50/50) composite samples. The polyolefins used were polyethylene (PE), polypropylene (PP), and a mixture of both PE and PP (50/50 w/w). The effects were studied in terms of the percentage water absorption, volumetric swelling efficiency, and mechanical and electrical properties. We found that the absorption of water and volumetric swelling were greatly retarded after 3 weeks in all of the wood flour/polyolefin composite samples containing various additives. It is also clear from the results that the mechanical properties were enhanced. The presence of ATH improved the electrical properties and enhanced the thermal stability of the wood flour composites. Generally, the PE composite samples gave better results compared to the PP ones. (c) 2008 Wiley Periodicals, Inc.