Ab initio HF/3-21G* and CIS/3-21G* calculations for the ground and the S-1 excited singlet states, respectively, of 2-(2＇-hydroxyphenyl)benzothiazole (HBT) and 2-(2＇-hydroxyphenyl)benzimidazole (HBI) were carried out. Geometric structures at the stationary points, including transition states, were investigated along with harmonic frequencies and hydrogen-bonding : characteristics. A topological analysis of the density function, rho, for the S-0 and S-1 states of the enol (N and N*) forms in the HEX series (X = -O-, -S-, and -NH-) provided some insight concerning the nature and trends of hydrogen bond interactions. The analysis of some parameters related to the intramolecular proton transfer suggests a qualitative relationship between the barrier height of the process and its exergonicity, consistent with principles of Leffler-Hammond and Bell-Evans-Polanyi and that changes in these two parameters are mainly determined by the strength of the hydrogen bond and to some extent by the electronegativity of X. Preliminary calculations carried out using continuum and specific models showed the important role of the specific effects of the solvent. Finally, some comments about dynamics were included.