(+)-Catechin and (-)-epicatechin are two flavonoid stereoisomers very abundant in vegetable tissues which present high antioxidant activity in living systems. It is known that the flavonoids exert their antioxidant properties according to the ability of trapping free radicals by donation of the phenolic hydrogen atoms. Nevertheless, the specific mechanism of the antioxidant activity remains far from well understood, due to the lack of information on the intrinsic molecular reactivity. The principal objective of this investigation was to study the molecular structure and chemical reactivity properties of (C)-catechin and (K)-epicatechin by analyzing the structural, chemical potential and energy properties, as well as Fukui indices, HOMO–LUMO distributions and 1 H– 13 C NMR spectrometries using the CHIH(medium)–DFT model chemistry. We found a similar reactivity in (C)-catechin and (K)-epicatechin, although the different sites for electrophilic attack showed by both molecules could mark the difference in the intermediates oxidation products formed, and therefore in their antioxidant ability. q 2006 Elsevier B.V. All rights reserved.