The variation of the ultimate stress and the elastic modulus of four different natural fiber-resin matrix composites was evaluated as a function of the aging of these composites in distilled water. Their diffusion coefficients and the maximum water content were also determined by fitting the experimental data to the Fickian model for diffusion. The composites analyzed were the common jute- and sisal-polyester matrix as well as jute- and sisal-epoxy resin matrix. The overall mechanical behavior observed could be qualitatively explained by the topographic aspects generated at the fracture surfaces. The jute-epoxy composite showed the best properties over all the immersion times analyzed. A strong fiber-matrix interface was developed for this composite and is partially responsible for its better behavior. The experimental data also show that the sisal fibers are more affected by water than jute fibers.