The standard (p degrees = 0.1 MPa) molar enthalpies of formation of 2-chloro-4-nitrophenol, 4-chloro-2-nitrophenol, and 4-chloro-3-nitrophenol in the solid phase, Delta H-f degrees(m)(cr), at T = 298.15 K, were derived from the standard molar enthalpies of combustion measured by rotating-bomb combustion calorimetry, in oxygen. Enthalpies of fusion of the solid compounds were measured by differential scanning calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, were measured by Calvet microcalorimetry. Summing the obtained Delta H-f degrees(m) (cr) and Delta H-g(cr)degrees(m) values, it was possible to derive the values of the standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K: Delta H-f degrees(m)(4-chloro-3-nitrophenol, g) = -(108.8 +/- 3.7) kJ.mol(-1). Density functional theory calculations were also performed for all possible chloronitrophenol isomers using the B3LYP functional and the 6-31G** basis set. These calculations allowed the estimation of the standard molar enthalpies of formation in the gaseous phase for all isomers including those not studied experimentally. Identical DFT calculations were also carried out for the respective phenoxyl radicals and phenoxide anions which allowed the estimation of the O-H homolytic bond dissociation energies and gas-phase acidities.