Graft copolymerization of styrene onto poly(vinyl chloride) (PVC) and polypropylene (PP) was carried out in a supercritical CO2 medium using AIBN as a free radical initiator. The supercritical CO2 medium served as a reaction medium in addition to being a solvent for the styrene monomer and the free radical initiator. The reaction temperature and pressure were kept above the critical points of the solvent-monomer mixture to form a homogeneous single-phase medium. The resulting graft copolymers were characterized using Fourier transform infrared spectroscopy FTIR), differential scanning calorimetry (DSC, thermogravimetric analysis (TGA), and nuclear magnetic resonance (NMR) techniques. The weight percent of grafting was determined using IR absorbance ratio technique. TGA results showed that the thermal stability of grafted copolymer of PVC was better than that of PVC, while grafted copolymer of PP had poorer thermal stability than PP. DSC results showed that glass transition temperatures (T-g's) of the grafted copolymers were higher than those of the starting polymers PVC and PP. I The presence of polystyrene attached to the backbone polymer was confirmed by H-1 NMR and C-13 NMR analyses.