Sisal fibers were incorporated into a mixture of benzoxazine and bisphenol A type epoxy resins to form a unidirectionally reinforced composite. Surface modifications of the sisal fibers were carried out with sodium hydroxide, gamma-aminopropyltrimethoxysilane, and gamma-glycidoxypropyltrimethoxysilane. The surface treatments led to changes in the morphology, chemical groups, and hydrophilicity of the fibers. The effect of the fiber surface treatments on the fiber-matrix interfacial adhesion and mechanical properties of the composites were also studied. The results showed that surface treatments with sodium hydroxide and a silane coupling agent led to improved fiber-matrix adhesion; this could be seen in the scanning electron micrographs of the fractured surfaces from mechanical testing and the reduction in the impact strength of the composites made from treated fibers. (c) 2007 Wiley Periodicals, Inc.