This work aims to study the influence of the amino group in positions 5 and 6 of the benzene ring of indazole. For that purpose, the standard (p degrees = 0 1 MPa) molar enthalpies of formation of 5- and 6-aminoindazole, in the gaseous phase, at T = 298.15 K were determined These values were calculated from the standard massic energies of combustion, measured by combustion calorimetry, and from the standard molar enthalpies of sublimation, computed from the variation with the temperature of the vapour pressures of each compound, measured by the Knudsen effusion technique. [GRAPHICS] The final results for the enthalpies of formation in the gaseous phase are discussed in terms of structural contributions of the amino group. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory The computed values compare very well with the experimental results obtained in this work and show that the 6-aminoindazole is the most stable isomer from the thermodynamic point of view. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies and, finally. N-H bond dissociation enthalpies (C) 2010 Elsevier Ltd All rights reserved