This work concerns the effects of gas type on the microcellular foam process when using atmospheric gases as a blowing agent. Three gases were investigated, CO2, N2 and argon. Gas absorption, viscosity reduction and cell formation were tested with these gases using the foaming process simulator (2) and an extruder with a capillary die. Significant differences were observed among these gases. The gas absorption of N2 and argon in polymers was much lower than CO2. The diffusion rate also seems to be lower with N2 and argon. The viscosity reduction was also lower with N2 and argon. Cell nucleation density was similar for all the three gases at high saturation pressures, but it was significantly lower with N2 or argon when the saturation pressure is low. The cell size is smaller with N2 and argon at a high saturation pressure, especially with N2. As a result, the foam density is higher with these two gases at the same foaming conditions compared to CO2.