This work is aimed at studying the foaming behavior of polypropylene (PP) with supercritical carbon dioxide (scCO(2)). The effects of saturation time, foaming temperature, saturation pressure and depressurization rate on the foam structure and volume expansion ratio were investigated. A likely foaming zone delimited by the upper and lower limits for foaming temperature and saturation pressure was determined. The upper limit for foaming was dictated by the melt strength of the PP while the lower one by the deformability of the PP. The maximum likely foaming temperature range was very narrow, only about 4 degrees C, whatever the saturation pressure, showing the poor foamability of PP. There was a sort of equivalence between increasing foaming temperature and increasing saturation pressure in terms of the foamability. However, a lower foaming temperature and a higher saturation pressure were more favorable for obtaining uniform foam than a higher foaming temperature and a lower saturation pressure. At given foaming temperature and saturation pressure, increasing depressurization rate led to an increase in cell density. However, both the cell size and the foam volume expansion ratio first increased with increasing depressurization rate, reached a maximum at some point and then decreased with a further increase in depressurization rate. (C) 2006 Elsevier B.V. All rights reserved.