B3LYP/6-31 1++G(2d,2p) 6d ionization potentials and O-H bond dissociation energies of natural cinnamic acids are consistent with an important antioxidant activity. The QTAIM analysis indicates that: (i) the benzene ring and the propenoic acid fragment of cinnamic acids behave as independent pi systems; (ii) the ionization process consists in a loss of nearly 1 a.u. of pi electron density by the atoms involved in HOMO accompanied by a reorganization of a electron density that is mostly provided by the remaining atoms. (c) 2006 Elsevier B.V. All rights reserved.