Composites with good mechanical properties were prepared from chemically modified soy oils and biofibers without additional petroleum-based polymers. These composites were prepared from maleic anhydride and epoxide functionalized soybean oils that were cured in the presence of various biofibers (e.g., kenaf, kayocell, protein grits, and solka-floc) by a flexible amine catalyst. Rigid thermosets characterized by a high-crosslink-density network and a high gel fraction were obtained. Fourier transform infrared was used to follow the cure reaction via the disappearance of the characteristic anhydride adsorptions. Composites with high tensile strength and low elongation were obtained when kenaf fibers were treated with (2-aminoethyl)-3-aminopropyl-trimethoxysilane and then added to the epoxidized/maleated soy matrix and cured with hexamethylenediamine. These biobased composites could provide inexpensive epoxy resin alternatives for a wide variety of industrial applications. (c) 2006 Wiley Periodicals, Inc.