Microcellular foams are well-known polymeric materials since they present excellent properties: high impact strength, high toughness, high stiffness-to-weight ratio, high fatigue life as well as low dielectric constant and low thermal conductivity. Environment-friendly techniques are being proposed to prepare polymeric foams. In particular, supercritical CO2 (scCO(2)) has received great attention instead of traditional blowing agents (chlorofluorocarbon hydrochlorofluorocarbons or hydrocarbons) due to its low cost and mild impact on the environment. In the foaming process proposed in this work, mild working conditions (90 bar and 30 degrees C) were required thanks to the presence of CO2 and p-cymene, which benefits the nucleation and cell growth. The influence of inorganic particles (silica and montmorillonite) on the cell morphology was studied experimentally and theoretically in order to promote the simultaneous formation of the embryos, thereby reducing the average cell size and narrowing the cell size distribution. A great enhancement of the polystyrene foam homogeneity was observed, independent of the nucleating agent employed and under low temperature conditions. [GRAPHICS] .