The foams of poly(ethylene terephthalate) (PET) and polycarbonate (PC) are prepared by rapid depressurization of CO2-saturated molten resin. It is found that the foamable temperature and pressure for PET and PC are nearly identical, and the attainable expansion ratio of the produced foam is generally less than 10. The effect of saturation temperature on the nucleation is mainly a result of the competition between the viscosity effect and the solubility effect. However, the effect of saturation pressure on the nucleation is twofold: the degree of supersaturation and the rate of depressurization. The effect of lowering the depressurization rate and changing the depressurization pattern on the polymeric foams is also investigated in this work. By careful observation of the microstructure, it is concluded that the foaming process is dominated by the nucleation kinetics. This study provides several alternative techniques for controlling the microstructure and the expansion ratio of PET and PC foams.