Supercritical fluids are being used in a wide range of applications, because they offer a number of unique physical, chemical and environmental advantages over other agents. Supercritical carbon dioxide (CO₂) is also being considered in the synthesis and processing of microcellular polymer foams as an alternative solvent. In this study, microcellular polystyrene (PS) foams were prepared by a solid-state batch process using supercritical or near-supercritical CO₂ as the foaming agent. The formation and morphology of the cellular structure were found to depend on the foaming process and process parameters, such as temperature, pressure, CO₂ soaking time, and depressurization rate. Two different foaming processes were investigated. In the one-step foaming process, the cellular structure was generated by pressure quenching of a CO₂-saturated PS sample. It was found that the cellular structure could be generated only under conditions with relatively high temperature, pressure and CO₂ concentration. In the two-step foaming process, pressure quenching is followed by soaking of the sample in a heat bath, which allows the saturated CO₂ to expand and create a cellular structure. The cellular structure created by the two-step process showed a higher porosity than that obtained with the one-step process. In addition, the two-step process enables the generation of cellular structures under low temperature and pressure conditions because of the secondary foaming process. The dependence of the cellular structure on the processing parameters for both processing methods was investigated and will be compared. This work was supported by Sealed Air Corporation and the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award Number DMR-0117792.