The effects of dissolved supercritical carbon dioxide on the viscosity and morphological properties were investigated for polyethylene/polystyrene blends in a twin-screw extruder. The viscosities of the blend/CO2 solutions were measured using a wedge die mounted on the extruder. A considerable reduction of viscosity was found when CO2 was dissolved in the blend. It was observed that the dissolution of CO, into PE/PS blends, regardless of the CO, content used, led to decreased shear thinning behavior resulting in an increase of the power law index from 0.29 to 0.34. The cell, structures of foamed PE/PS blends showed a typical dependence of pressure and CO, concentration, with higher operating pressures and CO, content leading to a smaller cell size. Also, it was noted that the size of the dispersed PS phase in the PE/PS phase blends decreased by increasing the CO2 concentration, and that the dispersed PS phase domains were highly elongated in the direction normal to the cell radius.