Many biopolymers and synthetic polymers composites were developed by different researchers for environmental protection and for cost reduction. One of these composites is polycaprolactone (PCL) and vital wheat gluten or wheat flour composites were prepared and compatibilized with polymeric diphenylmethane diisocyanate (pMDI) by blending and compression-molding. PCL/pMDI blend exhibited glass transition (T-g) at -67 degrees C (0.20 J/g/degrees C) and vital gluten at 63 degrees C (0.45 J/g/degrees C), whereas no T-g was recorded for wheat flour. Although T-g was unmistakable for either PCL or gluten, all composite exhibited one T-g, which is strong indication of interaction between PCL and the fillers. Several samples amongst the blended or compression-molded composites exhibited no T-g signifying another confirmation of interaction. The Delta H of the endothermic (melting) and the exothermic (crystallization) for PCL was decreased as the percentage of gluten or flour increased, whereas the overall Delta H was higher for all composites compared to the theoretical value. The presence of pMDI appeared to strengthen the mechanical properties of the composites by mostly interacting with the filler (gluten or flour) and not as much with PCL. The FTIR analysis ruled out covalent interaction between PCL, pMDI, or the fillers but suggested the occurrence of physical interactions. Based on the data presented here and the data published earlier, the presence of pMDI did not change the nature of interaction between PCL and gluten, but it improved the mechanical properties of the composite. (C) 2010 Wiley Periodicals, Inc.(dagger) J Appl Polym Sci 118: 2778-2790, 2010