This study examines the foaming behaviour of polystyrene (PS) blown with supercritical CO2-N-2 blends. This is achieved by observing their foaming processes in situ using a visualization system within a high-temperature/high-pressure view-cell. Through analyzing the cell nucleation and growth processes, the foaming mechanisms of PS blown with supercritical CO2-N-2 blends have been studied. It was observed that the 75% CO2-25% N-2 blend yielded the highest cell densities over a wide processing temperature window, which indicates the high nucleating power of supercritical N-2 and the high foam expanding ability of supercritical CO2 would produce synergistic effects with that ratio in batch foaming. Also, the presence of supercritical CO2 increased the solubility of supercritical N-2 in PS, so the concentration of dissolved supercritical N-2 was higher than the prediction by the simple mixing rule. The additional supercritical N-2 further increased the cell nucleation performance. These results provide valuable directions to identify the optimal supercritical CO2-N-2 composition for the foaming of PS to replace the hazardous blowing agents which are commonly used despite their high flammability or ozone depleting characteristics. (C) 2014 Elsevier B.V. All rights reserved.