The effect of the amount of sorbed water on the mechanical properties (tensile modulus, tensile strength, unit work of fracture, and characteristic strains) of composites based on a recycled low-density polyethylene, both unmodified and modified with diphenylmethane diisocyanate (DIC), is an a lyzed by statistical methods. The results of tensile tests are found to depend on the amount of sorbed water considerably. The elastic modulus, the unit work of fracture, and the characteristic strains correlate linearly with the amount of water. It is found that the elastic modulus drops after the sorption of water, but then, during the desorption process, it is restored gradually and reaches its initial value after a 30-day drying. This is explained by the plasticizing effect of water on composite materials containing hydrophilic natural fibers. DIC improves the interfacial interaction of the fiber-matrix interface and slows down the desorption of water. The investigations of the main deformational and strength characteristics of the unmodified and modified systems showed that the ad verse effect of water completely disappeared after a 30-day drying. The same conclusion, with a 95% probability, can be drawn from the results of an analysis of variance (ANOVA).