Carboxylated styrene-butadiene (SB) composites reinforced by a mixture of defatted soy flour (DSF) and carbon black (CB) were investigated in terms of their dynamic mechanical properties. DSF is an abundant renewable commodity and has a lower cost than CB. DSF contains soy protein, carbohydrate, and whey. Aqueous dispersions of DSF and CB were first mixed and then blended with SB latex to form rubber composites using freeze-drying and compression molding methods. At 140 degrees C, a single filler composite reinforced by 30% DSF exhibited roughly a 230-fold increase in the shear elastic modulus compared to the unfilled SB rubber, indicating a significant reinforcement effect by DSF. Mixtures of DSF and CB at three different ratios were investigated as co-fillers. Temperature sweep experiments indicate the shear elastic moduli of the co-filler composites are between that of DSF and CB composites. Strain sweep experiments were used to study the fatigue and recovery behaviors of these composites. Compared with the DSF composites, the recovery behaviors of the 30% co-filler composites after the eight consecutive deformation cycles of dynamic strain were improved and similar to that of 30% CB composite. Strain sweep experiments also indicated that the co-filler composites have a greater elastic modulus than the CB reinforced composites within the strain range measured. (C) 2007 Wiley Periodicals, Inc.