As part of an investigation of the role of tannin as antioxidants, the radical formation and radical decay reactions of some polyflavonoid and hydrolysable tannins has been followed by electron spin resonance (ESR) techniques. Comparative kinetics were determined for both light-induced radicals and by radical transfer from a less stable chemical species for the tannin alone and when the tannin is in a methanol solution. The five parameters which appear to have a bearing on the very complex pattern of the rates of tannin radical formation and radical decay were found to be (1) the extent of the colloidal state of the tannin in solution, (2) the stereochemical structure at the interflavonoid units linkage, (3) the ease of heterocyclic pyran ring opening, (4) the relative numbers of A- and B-rings hydroxy groups, and (5) solvation effects when the tannin is in solution. It is the combination of these five factors that appears to determine the behavior as an antioxidant of a particular tannin under a set of application conditions. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 475–482, 1997