A two-step depressurization batch process is developed to produce bi-modal cell structure polystyrene (PS) foams by using supercritical carbon dioxide (SC-CO2) as the blowing agent. Bi-modal cell structure foams can be achieved by depressurization in two distinct steps and can be significantly affected by the process parameters. The process conditions at the holding stage between the two steps are the key to controlling the bi-modal cell structure. The findings of this study can help predict the bi-modal cell morphology of PS and other polymers prepared by a two-step depressurization bath process. (C) 2010 Elsevier B.V. All rights reserved.