Biobased fillers were utilized as components in soy-elastomer composites. Soy flour is lightweight, low cost, and high strength, which makes it an ideal alternative to petroleum-derived fillers. However, poor interfacial adhesion and low dispersion within the polymer matrix are limiting factors for composite performance. Soy flour chemically pretreated by acetylation was compounded with synthetic rubber elastomers. In general, soy flour (as received) concentration in the composite is proportional to the ultimate strength. However, soy-elastomer composites with acetylated filler performed similar to the neat elastomer. In addition, the pretreated composite's thermal stability increased and exhibited less phase seperation compared to the untreated composites.