Assignment of C13 NMR bands to important sites of polymeric flavonoid tannin extracts produced industrially for production of tannin–formaldehyde resins is reported. Five different tannin extracts were studied. The C13 NMR bands' position and relative intensities allowed us to qualitatively explain comparative differences in the five tannins' structural features and properties that are of importance for tannin adhesives applications. The five tannins could be divided into two classes (1) mimosa and quebracho tannins and (2) pecan nut, pine, and gambier tannins. Properties of relevance that varied considerably among the five tannins and that could be detected by 13C NMR were: the relative proportions of fisetinidin/robinetinidin units in relation to catechin/gallocatechin units; differences in number average degree of polymerization; the extent of open heterocycle forms present; branching; and relative proportion of pyrogallol versus catechol B-rings in the flavonoid repeating units. © 1993 John Wiley & Sons, Inc.