The standard (p(o) = 0.1 MPa) molar enthalpies of formation for crystalline 3-methoxy-2-nitro-, 4-methoxy 3-nitro-, and 3-methoxy-4-nitrobenzoic acid were derived from the standard molar enthalpies of combustion, in oxygen, at the temperature 298.15 K, measured by static bomb combustion calorimetry, the Knudsen mass-loss effusion technique was used to measure the vapour pressures of these crystals as functions of temperature, and the standard molar enthalpies of sublimation, at T = 298.15 K, were derived by the Clausius-Clapeyron equation. The standard molar enthalpies of sublimation, at T = 298.15 K, for the three nitrobenzoic acids were also determined using the Knudsen mass-loss effusion technique. [GRAPHICS] The standard molar enthalpies of formation in the gaseous state are shown to fit a group additivity scheme.