Closed cell foams of broad molecular weight distribution commercial polystyrene samples, prepared by expansion of supercritical-CO2-swollen specimens, exhibit cell diameters that are similar to 3-10 times (19-24 mu m) larger than those of foams prepared from polystyrene samples with narrow molecular weight (NMW) distributions (similar to 2-6 mu m). Cell diameters for NMW samples are independent of molecular weight from 147K to 1050K Simulated polydisperse samples prepared by blending NMW distribution samples ranging from 560 to 1050K and a polydisperse sample prepared by radical polymerization produced foams with cells of the same size as in foams prepared from the NMW distribution samples. These observations suggest that molecular weight and polydispersity are not important factors in determining cell size and are not responsible for the disparity in cell sizes described above. This disparity is due to the presence of a very low molecular weight component (similar to 270) in the commercial samples. Extraction of this component reduced the cell diameter of resulting foams to that of the NMW distribution samples. Addition of a styrene oligomer (285) to a NMW distribution sample resulted in foams with larger cell diameters. Varying the concentration of this oligomer allows control of cell size in foams. Classical nucleation theory cannot explain these observations, suggesting that an alternative mechanism of cell formation is active.