Modification of wood fiber/flour (WF) surfaces can improve their compatibility with hydrophobic plastic matrices and reduce composite water uptake. WF was esterified with octanoyl chloride and palmitoyl chloride. Modified WF was analyzed by FT-IR. More extensive esterification Occurred in highly polar dimethylformamide (DMF) than in much less polar CHCl3 or methyl tert-butyl ether (MTBE). DMF penetrates into the fiber far more than CHCl3 or MTBE, making more -OH groups available for esterification. Increasing the acid chloride chain length from C-8 to C-16 decreased the mole fraction of esterification. Longer chains cover surface -OH groups, retarding reactions with nearby hydroxyls after esterification. Longer chain acid chlorides also have lower reactivity and penetrate into the hydrophilic wood fiber more slowly. Modified wood flour surfaces were covered by a hydrophobic layer of ester groups (SEM). Modified wood flour surfaces and WF/HDPE composite fracture surfaces were studied by SEM. C-8-modified wood flour (60 wt%)/HDPE composites exhibited far less water absorption after 24 h and 216 h immersions compared with unmodified WF (60 wt%)/HDPE composites. Water absorption continues over the 216 h period. Esterified WF/HDPE composites exhibited lower flexural strengths and moduli. In contrast to C-8-esterification, the addition of maleated polypropylene (MAPP) to WF/HDPE composites improved composite mechanical performance and gave similar water absorption properties to C-8-esterified WF composites. (C) Koninklijke Brill NV, Leiden, 2009