Filler mixtures of defatted soy flour (DSF) and carbon black (CB) were used to reinforce natural rubber (NR) composites and their viscoelastic properties were investigated. DSF is an abundant and renewable commodity and has a lower material cost than CB. Aqueous dispersions of DSF and CB were first mixed and then blended with NR latex to form rubber composites using freezedrying and compression molding methods. A 40% co-filler re nforcecl composite with a 1 : 1 DSF : CB ratio exhibited a 90-fold increase in the rubber plateau modulus compared with unfilled NR, showing a significant reinforcement effect by the co-filler. The effect, however, is lower than that observed in the carboxylated styrene-butadiene rubber composites reported earlier, indicating a significant effect from the rubber matrix. The co-filler composites have elastic moduli between those of DSF and CB reinforced composites. Stress softening and recovery experiments indicated that the co-filler composites with a higher CB content tend to have a better recovery behavior; however, this can not be simply explained from the recovery behaviors of the single filler (DFS and CB) composites. CB composites prepared by freeze-drying show a strain-induced reorganization of fillers. Strain sweep experiment data fit with the Kraus model indicates the co-filler composites with a higher CB content are more elastic, which is consistent with the recovery experiments. (c) 2007 Wiley Periodicals, Inc.