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, ΔfHm 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, Δcd gHm 0, was combined with the ΔfHm 0 (cd) in order to derive its gas-phase enthalpy of formation, ΔfHm 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 ΔfHm 0 (g) of 2 as (−1128.6 ± 6.4 kJ·mol−1), which was deduced from the isodesmic-reactions methodology. © 2018