The combined effects of alkali and ultrasound treatment of wood flour oil the moisture absorption behavior of polypropylene-based wood/plastic composites (WPCs) were examined. FTIR measurements confirmed that the alkali treatment removed both hemicellulose and lignin from the wood. and there was an increase in the number of hydroxyl groups on the cellulose surface. This process was promoted by ultrasound treatment. Particle size analysis of treated wood fibers revealed that alkali treatment reduced the particle size slightly, and the particle size was further reduced by the assistance of ultrasound. Moisture absorption measurements revealed the interplay between the presence of relative amounts Of Cellulose, hemicellulose and lignin in the wood and the amount of coupling agent added. It was found that appropriate alkali treatment and an optimum amount of added coupling agent could simultaneously improve composite mechanical properties and reduce both the rate and the maximum amount of moisture absorbed. Further improvements require preferential removal of hemicellulose from the wood and conditions that promote covalent bonding between the coupling agent and the wood reinforcement. (C) Koninklijke Brill NV, Leiden, 2009