A method for blending polystyrene and poly(methyl methacrylate), (PMMA), with the addition of supercritical carbon dioxide has been investigated. The first series of blends was a PMMA and polystyrene with similar melt viscosities. The second series of blends was a PMMA and polystyrene with a viscosity ratio (ηPMMA/ηpolystyrene) of about 20. The results show that a reduction in the size of the minor or dispersed phase is achieved when supercritical carbon dioxide is added to the blend system. A high-pressure mixing vessel was used to prepare the blends under pressure with carbon dioxide for batch blending. The solubilities of CO2 in PMMA and polystyrene, measured in the high-pressure mixing vessel at 200°C and 13.78 MPa (2000 psi) was 5.8 and 3.6 wt%, respectively. A single screw extruder was used to study the effects of carbon dioxide on the viscosity of polymer melts. The melt viscosity of PMMA was reduced by up to 70% with approximately 0.4 wt% CO2. The melt viscosity of polystyrene was reduced by up to 56% with a CO2 content of 0.3 wt%. A twin screw extruder was used to study the effects of injecting carbon dioxide in a continuous compounding operation.