The effects of the bark content on the water absorption and thickness swelling of wood-plastic composites prepared from polypropylene, wood flour, and bark flour were studied. Samples were made with a laboratory twin-screw extruder. The results showed that among composites free of maleic anhydride polypropylene, those composites containing a higher bark flour content exhibited lower water absorption and lower thickness swelling. Maleic anhydride polypropylene reduced water absorption and thickness swelling in composites containing wood flour and a lower content of bark flour but had no influence on the hygroscopic properties of composites made with higher bark contents. Adding maleic anhydride polypropylene had no effect on the water diffusion coefficients and swelling rate parameters of composites made with a higher bark flour content. (c) 2008 Wiley Periodicals, Inc.