We report a study on the energetics and structural properties of gallic (1) and ellagic (2) acids. The experimental values of standard enthalpy of formation in solid state at 298.15 K, Delta H-f(m)0 (cd) of 1 as (-985.0 +/- 2.9 kJ.mol(-1)) and 2 as (-1377.9 +/- 4.7 kJ.mol(-1)) have been determined. The vapour pressure of 1 have been measure by Knudsen effusion methodology and the derived enthalpy of sublimation, Delta H-g(cd)m(0). was combined with the Delta H-f(m)0 (cd) in order to derive its gas-phase enthalpy of formation, Delta(f) H-m(0)(1,g) = -835.7 +/- 4.0 kJ.mol(-1). Quantum chemical calculations, at DFT (M05-2X) and composite ab initio Gn (n = 3, 4) levels of theory, provided the consistency of the experimental results and a plausible estimation of Delta H-f(m)0 (g) of 2 as (-1128.6 +/- 6.4 kJ.mol(-1)), which was deduced from the isodesmic-reactions methodology. (C) 2018 Published by Elsevier Ltd.