The foaming of poly(butylene succinate) (PBS) using supercritical CO2(scCO2) was studied. In order to improve the melt strength, PBS was modified using the reactive compounding technique. Rapid decompression of scCO2-saturated PBS at a temperature above the depressed Tm yielded expanded microcellular foams. The resulting foam structure could be controlled by manipulating process conditions. Experiments varying the foaming temperature while holding other variables constant showed that higher temperatures produced larger cells and reduced cell densities. Higher saturated pressures led to higher nucleation densities and smaller cell sizes. Decreasing the rate of depressurization permitted a longer period of cell growth and therefore larger cells were obtained.