The standard ( p ∘ = 0.1 MPa) molar enthalpies of formation of 2-, 3-, and 4-chloronitrobenzene isomers, in the crystalline state, at T = 298.15 K, were derived from the standard ( p ∘ = 0.1 MPa) massic energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of sublimation of the isomers, at T = 298.15 K, were obtained by high temperature Calvet microcalorimetry. - Δ c U m ∘ ( cr ) /(kJ · mol −1) - Δ c H m ∘ ( cr ) /(kJ · mol −1) - Δ cr g H m ∘ /(kJ · mol −1) 2-Chloronitrobenzene 2939.5 ± 0.7 18.7 ± 1.0 80.9 ± 1.5 3-Chloronitrobenzene 2915.8 ± 0.8 42.4 ± 1.2 82.5 ± 1.5 4-Chloronitrobenzene 2921.7 ± 1.4 36.5 ± 1.6 76.2 ± 2.1 From the determined experimental data, the values of the gaseous standard ( p ∘ = 0.1 MPa) molar enthalpies of formation for the three monochloronitrobenzene isomers were derived. The gas-phase enthalpies of formation were also estimated by the empirical scheme developed by Cox showing that for meta- and para-chloronitrobenzene the estimated values are in close agreement with the experimental ones whereas, in the case of ortho-chloronitrobenzene it is shown that a different enthalpic interaction increment is needed, when the substituents in the adjacent carbon ring atoms are a chlorine atom and a nitro group.