The foaming process of poly(e-caprolactone) (PCL) with carbon dioxide and nitrogen has been investigated in this work from a theoretical and experimental point of view. CO2 and N-2 showed very different behavior, as foaming agents for PCL. This was due to the different transport, chemical, and physical properties of the polymer/gas mixture that led to different foam morphology in terms of density, cell number density, and cell size. The lowest density (0.03 g/cm(3)) was obtained with CO2, but the highest number of cells with N-2 (although with a higher density, (0.2 g/cm(3)). Foam with a low-density microcellular structure, was obtained when a mixture of the two gases was employed. (c) 2005 Society of Plastics Engineers.