A series of biodegradable plastics from soy protein isolate (SPI) and lignosulfonate (LS) with a weight ratio of 0:10 to 6:4 were prepared with 40 wt % glycerol as a plasticizer by compression molding. Their properties were investigated by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), dynamical mechanical thermal analysis (DMTA), scanning electron microscopy (SEM), and tensile tests. The results indicated that the introduction of a moderate LS content from 30 to 40 parts in the blends could simultaneously enhance the tensile strength, elongation, and Young's modulus of soy protein plastics alone. Studies of the water sensitivity of the materials suggested that the strong interaction between LS and SPI could restrict the effect of water on the swelling and the damage of the materials, resulting in lower water absorption. The improvement of the properties was attributed mainly to the existence of the beneficial microphase separation and the formation of crosslinked structures because of the introduction of LS into soy protein plastics. Therefore, a model of a crosslinked network formed from SPI molecules with an LS center was established based on the existence of strong physical interactions between LS and SPI. (C) 2003 Wiley Periodicals, Inc.