In the present work, the values of the standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of 1,2-diaminobenzene, 1,3-diaminobenzene, and 1,4-diaminobenzene are reported as 86.6 +/- 1.6, 89.6 +/- 1.6, and 99.7 +/- 1.7 kJ.mol(-1), respectively. These values were derived from experimental thermodynamic parameters, namely the standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, Delta(f) H-m degrees, (cr), at T = 298.15 K, obtained from the standard molar enthalpies of combustion, Delta(c) H-m degrees, measured by static bomb combustion calorimetry, and the standard molar enthalpies of sublimation, at T = 298.15 K,derived from the temperature-vapor pressure dependence, determined by the Knudsen mass loss effusion method. The results were compared with estimates obtained by standard ab initio molecular calculations at the G3(MP2)//B3LYP level. Experimental and calculated data are in very good agreement and show that the 1,2-diaminobenzene is, thermodynamically, the most stable isomer. Finally, proton and electron affinities, basicities and adiabatic ionization enthalpies were also computed at the same level.