Polyflavonoid tannin-formaldehyde-furfuryl alcohol rigid foams yield hardened, rigid, tridimensional networks in which there are covalently linked structures derived from the reaction of their main components. Thus, polyflavonoid tannin structures, tannin-formaldehyde structures, furanic structures derived by the self-condensation of furfuryl alcohol, and mixed tannin-furane and tannin-furane-formaldehyde structures are all present in the continuous foam networks. Some complex, tridimensional structures involving tannin's flavonoid units, furan nuclei, and formaldehyde-derived methylene bridges appear to be formed too. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry has been used to determine the different segments contributing to forming the complex thermosetting networks of the hardened foams. (C) 2008 Wiley Periodicals, Inc.