Effects of surface treatments on the strength and water resistance of kenaf fiber-reinforced unsaturated polyester (UPE) composites were investigated. A new coupling agent that consists of 1,6-diisocyanatohexane (DIH) and 2-hydroxylethyl acrylate (HEA) was investigated for surface treatments of kenaf fibers. The surface treatments were found to significantly enhance the tensile strength, modulus of rupture, modulus of elasticity, and water resistance of the resulting kenaf-UPE composites. Fourier transform infrared spectroscopy (FTIR) confirmed that DIH-HEA was covalently bonded onto kenaf fibers. Scanning electron microscopy (SEM) images of the composites revealed that chemical treatment of kenaf fibers with a combination of DIH and HEA improved the interfacial adhesion between kenaf fibers and UPE resin in the DIH-HEA-treated kenaf-UPE composites. The mechanisms by which the chemical treatment of kenaf fiber surfaces improved strength and water resistance of the resulting kenaf-UPE composites were discussed. (C) Koninklijke Brill NV, Leiden, 2012