Microcellular polymeric foam structures have been generated using a pressure induced phase separation in concentrated mixtures of supercritical CO2 and SAN. The process tyically generates microcellular core structure encased by nonporous skin. Pore growth will occur through two me mechanisms: diffusion of CO2 from polymer-rich regions into the pores and also through CO2 gas expansion. The effects of saturation pressure, temperature, and swelling time on the cell size, cell density, and bulk density of the porous materials have been studied. Higher CO2 pressure lowered polymer viscosity and interfacial tension between SAN and CO2 to produce cells with smaller average size but higher cell density, resulted from increase inswollen CO2 into SAN matrix. This trend was similar to what was observed in swelling time series. While the average cell size increased with increasing temperature, the cell density decreased. The trend of bulk density was similar to that of cell size.