The enthalpy of combustion and enthalpy of sublimation of 3,4,5-trimethoxybenzoic acid (2) were measured, and the gas-phase enthalpy of formation at 298.15 K was calculated. Ab initio energies at the B3LYP/ 6-311+G(d,p) level were calculated for this compound, some mono- and dimethoxybenzoic acids, and a similar series of methyl-substituted benzoic acids. Intramolecular interaction (called the buttressing effect) of the groups COOH and OCH3 (or CH3) was defined in terms of isodesmic homodesmotic reactions. Steric crowding raises the energy of 2 in relation to monomethoxybenzoic acids, but no further effect is observed in comparison with 3,4-dimethoxybenzoic acid, on either calculated or experimental values. Similar results were obtained with 3,4,5-trimethylbenzoic acid (1); a previous claim of interaction of remote groups in the meta position was based on inaccurate enthalpies of formation of reference compounds.