Methylene diphenyl diisocyanate (MDI) was used to compatibilize kraft lignin (KL)/soy protein isolate (SPI) blends. The structure and properties of the resultant composite materials were investigated with wide-angle Xray diffraction, differential scanning calorimetry, dynamical mechanical thermal analysis, scanning electron microscopy, and tensile and water absorption tests. The results indicated that graft copolymerization and a moderate degree of crosslinking between KL and SPI occurred in the composites because of the compatibilization of MDI, which favored the strengthening of the materials. Interestingly, the addition of 2 parts of MDI caused a simultaneous enhancement of the modulus, strength, and elongation of KL/SPI blends. The structure with grafting and moderate crosslinks reduced the water absorption of the materials. However, the excess crosslinks hindered the interaction between KL and SPI, resulting in a reduction of the mechanical properties. Scanning electron microscopy showed that the domains of the graft copolymer and crosslinking enrichment existed in the blends. When the MDI content was relatively low, these domains became concentric points of stress, enhancing the mechanical properties. (C) 2004 Wiley Periodicals, Inc.