Starch microcellular foam was produced by supercritical fluid extrusion (SCFX) using native and pregelatinized starch mixtures. The starch solution was reacted with epichlorohydrin (EPI) under alkaline conditions in a continuous twin-screw extruder in the presence of supercritical carbon dioxide. The relationship between crosslinking density and cell size and their distribution in the foamed extrudates was studied. An increase in the EPI concentration from 0 to 0.5% was accompanied by an increase in the degree of crosslinking as measured by differential scanning calorimetry and confirmed by diffusion coefficient measured by nuclear magnetic resonance. The diffusion coefficient of crosslinked sample was observed to be 12.5 times lower than that of non crosslinked control. The average cell size of the microporous structure, observed by scanning electron microscopy, showed a decrease from 147 to 61 mu m as the concentration of EPI was increased. The cell size distribution of the crosslinked samples was significantly improved but the total expansion was lowered. The results obtained to date indicate, for the first time, that reactive extrusion of starch with EPI with supercritical carbon dioxide as a blowing agent offers a promising new technique to generate microporous foams for use in various applications. (C) 2007 Wiley Periodicals, Inc.