The aggregate structure of phthalic anhydride (PA) modified soy protein isolate (SPI) was investigated by estimating its fractal dimension from the equilibrated dynamic strain sweep experiments. The estimated fractal dimensions of the filler aggregates were less than 2, indicating that these particle aggregates have a distorted or broken two-dimensional sheet-like structure. The results also indicated that the aggregate structure has a greater effect on the composite reinforcement than the overall aggregate size. Tensile strength, elongation, Young's modulus, and toughness of hydrolyzed/modified soy composites are comparable with those of carbon black reinforced composites at 10-15% filler fraction. The moduli of PA-modified SPI composites were less sensitive to the pH of the composite preparation compared to the unmodified SPI. The composites prepared at acidic pH, with lower filler fraction, or filled with hydrolyzed/modified SPI are more elastic and less fatigue. The composites of PA-modified SPI had better recovery properties when prepared at acidic instead of alkali pH. PA-modified hydrolyzed SPI composites prepared at acidic pH showed a similar recovery property to that of carbon black reinforced composites, but with greater shear elastic moduli. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 1992-2001, 2011