Nearly all rubber used today utilizes filler materials for strength and various other application-specific properties. The most common filler on the market today is carbon black, which is produced by the burning of petroleum. Using renewable biomaterials as fillers would reduce dependence on petroleum and avoid pollution issues associated with carbon black production. Recent research using various soybean materials such as defatted soy flour, soy spent flakes, and soy protein isolate has shown that fillers that compare favorably in both performance and cost characteristics can be made from these products. In general, the smaller the filler particle size, the better strengthening properties it has. In this study, variables including temperature, pH, and centrifugation were all examined to determine what conditions produced the smallest soy protein isolate particles for filler material.