The miscibility of poly(vinyl chloride)/natural rubber-graft-polyacrylonitrile (PVC/NRPACN) and PVC/NR-graft-poly(methyl methacrylate) (PMMA) were studied by differential scanning calorimetry (DSC) and viscometry. DSC curves of PVC/NR graft copolymer blends showed a major endotherm at about 265 degrees C that was assigned to the thermal decomposition of PVC. PVC/NRPMMA blends showed a minor endothermic peak at about 124 degrees C and the PVC/NRPACN blends showed a broad peak (which develops into two peaks) at about 160 degrees-180 degrees C, respectively. Absolute viscosity versus weight percent PVC plots of the two sets of blends were nonlinear. X-ray diffraction patterns of the PVC/NRPACN blends showed overlapping spacings due to the constituent polymers. Phase-contrast micrographs of the PVC/NRPACN blends showed gross phase discontinuity. The heats of mixing solutions of PVC blended with NRPMMA were found to be well above the upper limit of miscibility.