A theoretical study on antioxidant properties of fourteen xanthones extracted from the pericarp of G. Mangostana has been performed. Three main reaction mechanisms are investigated: hydrogen atom transfer (HAT), single electron transfer-proton transfer (SETPT) and sequential proton loss electron transfer (SPLET). The O-H bond dissociation enthalpy (BDE), ionization energy (IE), proton affinity (PA) and electron transfer energy (ETE) parameters were computed in gas phase and water. The results show that HAT would be the most favorable mechanism for explaining antioxidant activity of xanthones in gas phase, whereas the SPLET mechanism is thermodynamically favored in water. (C) 2015 Elsevier B.V. All rights reserved.