Theoretical and experimental studies on the energetic, structural and some other relevant physicochemical properties of the antioxidant tyrosol (1), hydroxytyrosol (1OH) molecules and the corresponding radicals 1(rad)(center dot) and 1O(rad)(center dot) are reported in this work. The experimental values of the gas -phase enthalpy of formation, Delta H-f(m)0(g), in kJ.mol(-1), of 1 (-302.4 +/- 3.4) and 1OH (-486.3 +/- 4.1) have been determined. Quantum chemical calculations, at DFT (M05-2X) and composite ab initio G3 and G4 levels of theory, provided results that served to (i) confirm the excellent consistency of the experimental measurements performed, (ii) establish that the stabilizing effect of H -bond of hydroxyethyl chain and aromatic ring (OH center dot center dot center dot pi interaction) is smaller in radicals than in parent molecules, (iii) deduce-combining experimental data in isodesmic reactions-Delta H-f(m)0(g) of radicals 1(rad)(center dot) (-152.3 +/- 4.4 kJ.mol(-1)) and 1O(rad)(center dot) (-370.6 +/- 3.8 kJ.mol(-1)), (iv) estimate a reliable O-H bond dissociation enthalpy, BDE of 1 (368.1 +/- 5.6 kJ.mol(-1)) and of 1OH (333.7 +/- 5.6 kJ.mol(-1)), and (v) corroborate-using "BDE criteria"-than 1OH is a more effective antioxidant than 1.