Supercritical carbon dioxide (scCO(2)) was added during blending of polystyrene or poly(methyl-methacrylate) (PMMA) and a rubber impact modifier (SP 2207). The resulting blend morphologies were compared. The compounding took place in a Leistritz ZSE-27 twin-screw extruder at 100 RPM, at a temperature of 200 degreesC, and with 2.0 wt% CO2 injection. The viscosity reduction of PMMA, polystyrene, and SP 2207 was measured using a slit die rheometer attached to the twin-screw extruder. A viscosity reduction of up to 84% was seen with PMMA, 70% with polystyrene and 30% with SP 2207. The solubility of CO2 in these polymers was measured in a high-pressure vessel at 200 C and 13.78 MPa (2000 psi). A solubility of 5.79 wt% CO2 was seen with PMMA, 3.65 wt% with polystyrene, and 2.60 wt% with SP 2207. The injection of CO2 reduced the size of the dispersed rubber phase in both polystyrene and PMMA. For both blends (polystyrene/SP 2207 and PMMA/SP 2207) with and without the injection of CO2, the extruder length for phase inversion was shortened by about L/D = 4, or 10% of the total extruder length. The impact strength for a 70/30 polystyrene/SP 2207 blend was increased by 26% by the addition of CO2. The improvement in impact strength was not as large for blends of PMMA and SP 2207.