A study on the extrusion of polystyrene was carried out using supercritical carbon dioxide (scCO(2)) as foaming agent. scCO(2) modifies the theological properties of the material in the barrel of the extruder and acts as a blowing agent during the relaxation at the passage through the die. For experiments, a single-screw extruder was modified to be able to inject scCO(2) within the extruded material. The effect of operating parameters on material porosity was studied. Samples were characterized by using water-pycnometry, mercury-porosimetry and scanning electron microscopy. Polystyrene with expansion rate about 15-25% was manufactured. A rapid cooling just downstream the die is important to solidify the structure. The die temperature allows the control of the porosity structure. CO2 concentration shows no significant influence. (C) 2007 Elsevier B.V. All rights reserved.