Foaming of polystyrene by previous dissolution in p-cymene is studied using CO2 to saturate the solution followed by a rapid decompression of samples. Although during the saturation stage the polymer is plasticized and the glass transition temperature is decreased, the main advantage offered by using a solvent is the reduction of the working temperature. The effects of pressure, temperature, and initial concentration of the polymer solutions on cell size, distribution of pores, and cell density of the foams are analyzed for optimization of the foaming process in order to achieve microcellular structures. Optimum conditions were achieved at high values of pressure and concentration and low temperature.