The proton affinity and gas-phase basicity of 14 heterocyclic aromatic compounds containing two or more nitrogen atoms are investigated in this work. Strong linear correlations of these quantities with the molecular electrostatic potential on the nitrogen nuclei and natural valence orbital energies were observed. We justified the relationships under the framework of density functional reactivity theory as the first-order approximation. These linear relationships suggest that the associating proton prefers to bind with the basic atom with the lowest electrostatic potential value. Different density functional formulas and basis sets have been employed to verify the validity of these results. (C) 2012 Elsevier B. V. All rights reserved.