The reactions between furfuryl alcohol (FA)/poly(furfuryl alcohol) (PFA) and hexamethylenetetramine (HMTA) have been studied by C-13 and N-15 high-resolution solid-state NMR techniques. Highly crosslinked polymer networks similar to those obtained from the thermal curing of FA resins under acidic condition are formed. Possible reaction pathways are postulated on the basis of changes of chemical structures during the curing; alpha-substituted furfurylamines are shown to be the initial intermediates. Their further reactions with FA/PFA, together with thermal decomposition, produce methylene linkages between furan rings, resulting in chain extension and crosslinking, which occurs at both the methylene linkages and the 3- or 4-C of furan rings. Various side-products such as amines, imines, amides, imides, and nitriles are also formed during the reactions, and some of these can remain in the resins up to 205 degrees C. The crosslinked network can be heterogeneous with different structures on nanometer scales when higher oligomers of FA resins react with HMTA.