Microcellular polystyrene (PS) foams and porous structures of the biodegradable poly(D,L-lactic acid) (P(D,L)LA) were prepared with the batch foaming technique (pressure quench) using supercritical CO2 as blowing agent. The effect of pressure, temperature and depressurization rate on the final porous structure was investigated. The results revealed that the size of the pores decreases and their population density increases with pressure increase, or decrease of temperature, and/or increase of the depressurization rate. The results were correlated by combining nucleation theory with NRHB model in order to account for and emphasize the physical mechanism related to nucleation of bubbles inside the supersaturated polymer matrix. A satisfactory agreement between correlations and experimental data was obtained indicating that the nucleation theory yields quantitative correlations when variables such as sorption, degree of plasticization, and surface tension of the system polymer-supercritical fluid are accurately described. (c) 2007 Elsevier Ltd. All rights reserved.