Poly(vinyl chloride)/polystyrene (PVC/PS) blends were prepared by using supercritical CO2 as a swelling agent, and by thermally polymerizing styrene within the PVC matrix. It could be demonstrated by means of scanning electronic microscopy that PS was dispersed uniformly in the PVC matrix with very fine phase size. According to differential scanning calorimetry measurements and dynamic mechanical analysis, each blend had one single glass transition temperature, which indicated that compatible blends were obtained.