The energetics of the phenolic O-H bond in the three hydroxybenzoic acid isomers and of the intramolecular hydrogen O-H- - -O-C bond in 2-hydroxybenzoic acid, 2-OHBA, were investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of monoclinic 3- and 4-hydroxybenzoic acids, at 298.15 K, were determined as Delta(f)H(m)degrees(3-OHBA, cr) = -593.9 +/- 2.0 kJ(.)mol(-1) and Delta(f)H(m)degrees(4-OHBA, cr) = -597.2 +/- 1.4 kJ(.)mol(-1), by combustion calorimetry. Calvet drop-sublimation calorimetric measurements on monoclinic samples of 2-, 3-, and 4-OHBA, led to the following enthalpy of sublimation values at 298.15 K: Delta(sub)H(m)degrees(2-OHBA) = 94.4 +/- 0.4 kJ(.)mol(-1), Delta(sub)H(m)degrees(3-OHBA) = 118.3 +/- 1.1 kJ(.)mol(-1), and Delta(sub)H(m)degrees(4-OHBA) = 117.0 +/- 0.5 kJ(.)mol(-1). From the obtained Delta(f)H(m)degrees(cr) and Delta(sub)H(m)degrees values and the previously reported enthalpy of formation of monoclinic 2-OHBA (-591.7 +/- 1.3 kJ(.)mol(-1)), it was possible to derive Delta(f)H(m)degrees(2-OHBA, g) = -497.3 +/- 1.4 kJ(.)mol(-1), Delta(f)H(m)degrees(3-OHBA, g) = -475.6 +/- 2.3 kJ(.)mol(-1), and Delta(f)H(m)degrees(4-OHBA, cr) = -480.2 +/- 1.5 kJ(.)mol(-1). These values, together with the enthalpies of isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3PW91/aug-cc-pVDZ, MPW1PW91/aug-cc-pVDZ, and MPW1PW91/aug-cc-pVTZ) and the CBS-QMPW1 methods, were used to derive the enthalpies of formation of the gaseous 2-, 3-, and 4-carboxyphenoxyl radicals as Delta(f)H(m)degrees(2-HOOCC6H4Ocenter dot, g) = -322.5 +/- 3.0 kJ(.)mol(-1) Delta(f)H(m)degrees(3-HOOCC6H4Ocenter dot, g) = -310.0 +/-3.0 kJ(.)mol(-1), and Delta(f)H(m)degrees(4-HOOCC6H4Ocenter dot, g) = -318.2 +/- 3.0 kJ(.)mol(-1). The O-H bond dissociation enthalpies in 2-OHBA, 3-OHBA, and 4-OHBA were 392.8 +/- 3.3, 383.6 +/- 3.8, and 380.0 +/- 3.4 kJ(.)mol(-1), respectively. Finally, by using the ortho-para method, it was found that the H- - -O intramolecular hydrogen bond in the 2-carboxyphenoxyl radical is 25.7 kJ(.)mol(-1), which is ca. 6-9 kJ(.)mol(-1) above the one estimated in its parent (2-OHBA), viz. 20.2 kJ(.)mol(-1) (theoretical) or 17.1 +/- 2.1 kJ(.)mol(-1) (experimental).