In this work, we present experimental and theoretical thermochemistry of the 3- and 4-nitrophtalic acids. We report their standard molar enthalpies of formation in crystalline phase, at T = 298.15 K, which were obtained from their energies of combustion. The latter were determined through an isoperibolic bomb calorimeter. We measured the change-phase enthalpies, as well as the fusion and sublimation enthalpies, by differential scanning calorimetry (DSC) and thermogravimetry (TG). Also via DSC, we found the heat capacity equations (in crystalline phase) for both isomers as functions of temperature. We determined the enthalpies of formation in both crystal-and gas-phase from the previous measurements. In addition, we computed the enthalpies of formation in gas-phase of both isomers using the composite Gaussian-G4 method and atomization reactions. The final theoretical enthalpies were computed using a weighted Boltzmann averaging procedure, and the weights were estimated using the Gibbs free energy at T = 298.15 K. The absolute differences between theoretical and experimental enthalpies are below 3.3 kJ.mol (-1). Finally, we apply our results for obtaining the isomerization enthalpy of 3- to 4-nitrophthalic acid, and difference enthalpies between the nitrophthalic acids and their anhydrides. (C) 2018 Elsevier Ltd.