Sponge gourd (Luffa cylindrica) forms a natural mat that deviates the crack path in brittle thermoset resin matrix composites, leading to a controlled fracture mode and increasing the toughness of the composite. The use of luffa as reinforcement is, however, restricted by a very weak fiber-matrix interface. In this work, luffa fibers were alkali-treated at two temperatures, with varying alkali concentrations. Although the surface analysis shows that the treatments promote a clear removal of the outer surface layer of the fibers with the exposition of the inner fibrillar structure and the consequent increase of the fiber surface area, only a secondary increase on the mechanical properties was obtained. The slight increase observed was attributed only to mechanical interlock. Long pullout lengths and neat fiber beads were the main features observed at the fracture surface of the treated and untreated fiber composites. (C) 2003 Wiley Periodicals, Inc.