Ionisation potentials and O-H bond dissociation energies of natural hydroxybenzoic acids are consistent with an important antioxidant activity. All the atomic electron populations, but that of the carbonylic oxygen, vary during the ionisation processes in contrast to what could be predicted from the distribution of the HOMO. The most important part of the electron charge loss is removed from hydrogens, and is transferred to the carbons of the ring compensating their loss of 71 charge. O-H bond dissociation energies are not correlated with the electron densities at the corresponding bond critical point. (C) 2004 Elsevier B.V. All rights reserved.