The linear and nonlinear viscoelastic properties of natural rubber composites reinforced with defatted soy flour (DSF) were studied. DSF is an abundant, renewable commodity, and its rigid nature makes it Suitable as a reinforcement phase in rubber composites. At small strain, the elastic modulus of a 30% filled composite was about 20 times higher than the unfilled rubber. Greater reinforcement was observed for carbon black filled composites than for DSF filled composites at filler concentrations of 10 and 20%. At high strain, values of the relaxation Modulus dropped more rapidly for highly filled DSF composites. At high shear rates in a capillary viscometer, a small reinforcement effect remained for all composites, and lower die swell was observed for DSF composites. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2049-2055, 2009