Genistein has been employed to modify epoxidized soybean oil (ESBO) plasticized poly(vinyl chloride) (PVC) membranes for improved biomedical applications. The binary blends consisting of ESBO/genistein (ESBO/G), PVC/ESBO, and PVC/G pairs have been investigated using differential scanning calorimetry and polarized optical microscopy. PVC/ESBO blends were found to be miscible, whereas PVC/genistein blends were immiscible. ESBO/G blends exhibited miscibility at low concentrations of genistein showing single glass transition. At high genistein concentrations, crosslinking reaction occurred below the melting temperature of the genistein crystals. The ternary PVC/ESBO/G blends were miscible in the amorphous phase at lower loading of genistein implying that the ESBO effectively compatibilized the otherwise immiscible PVC/genistein pair. With the addition of ESBO, the thermal stability of PVC was found to improve in the binary mixture as well as in the ternary blends with genistein. The thermal curing mechanism between ESBO and genistein was explored using Fourier transform infrared spectroscopy (FTIR). The FTIR spectra suggested the occurrence of curing reaction between the epoxy groups of ESBO and the hydroxyl groups of genistein. Subsequently, the effect of curing was investigated as a function of ESBO concentration using dynamic mechanical analysis. A systematic movement of the storage modulus and loss tangent peak (corresponding to Tg) of the PVC membrane to a lower temperature due to the ESBO addition was seen, but this relaxation peak was shifted to a higher temperature upon thermal curing. In the anti-oxidant study, the genistein modified PVC/ESBO membranes exhibited significant reductions of the reactive oxygen species upon contact with blood. Acknowledgement: Support of this work by the Ohio Soybean Council is gratefully acknowledged.