The foaming of thin (~100 m) polysulfone (PSU), poly(ether sulfone) (PES), and cyclic olefin copolymer (COC) films using carbon dioxide as a physical blowing agent has been studied. Microcellular foam morphologies were obtained by saturating the polymer with carbon dioxide and heating the sample above the glass transition temperature of the polymer/gas mixture after releasing pressure. The temperature range at which foaming took place was examined in detail, and the physical processes fixing the final foam morphologies were discussed. We find that the ease of plasticization of the different polymers and the CO2 diffusion coefficient under foaming conditions determine the morphology of the foams. Nucleation and growth of cells starts at the Tg of the polymer/gas mixture; however, this process is severely inhibited by enhanced diffusion of gas from the films. The maximum cell density attainable is not determined by the ease of nucleation. Instead, CO2 loss by diffusion to the exterior of the sample determines the maximum number of cells in the polymer. This effect, which at sufficiently high temperatures results in decreasing cell densities with increasing temperature of the foaming bath, becomes stronger for the more readily plasticized system (COC > PSU > PES) and causes an upper temperature limit where foaming stops.