The vegetable fibers used for polymer matrix composites are usually treated to improve their adhesion with the matrix. The chemical treatment with sodium hydroxide (NaOH), although widely used, may damage the fiber surface structure, reducing its strength. The possibility of protecting vegetable fibers against alkaline chemical aggression by using hydride ions (H-) was investigated in this work. Sisal fibers were modified by immersion in a NaOH aqueous solution (2, 5 and 10% wt/vol), with or without the addition of sodium borohydride (NaBH4) (1% wt/vol), under variable conditions (immersion time and temperature). The effect of using NaBH4 was investigated using fiber tensile and pull-out tests, critical length calculation, along with a Weibull statistical analysis. This agent was found to minimize sisal degradation under highly concentrated alkaline conditions in comparison with sisal treated with the pure NaOH solution. The results suggest the 5% wt/vol treatment for 60 min under room temperature in the presence of the hydride ions as the most suitable for sisal. This result may be extended to other vegetable fibers of similar composition and may promote their use in polymer composites. (C) Koninklijke Brill NV, Leiden, 2011