Tautomerism and resonance-assisted hydrogen bonding have been analyzed on the basis of the results of ab initio calculations at the MP2/AUG-cc-pVDZ level of theory for the series of molecules containing different heterocycles connected with resonance spacer and containing different exocyclic proton donor/acceptor atoms. It is demonstrated that the position of tautomeric equilibrium is controlled mainly by two factors: aromaticity of heterocycle, which could be different for two tautomers, and relative proton affinities of two heteroatoms forming a hydrogen bond. Replacement of exocyclic proton donor/acceptor atom result; in change of an aromaticity degree of heterocycle leading to alteration of relative stability of tautomers. Comparison of structure and properties of E and Z conformers of molecules demonstrates resonance-assisted character of intramolecular hydrogen bond. Application of the NBO theory reveals that the pi-component of the electron density within resonant spacer plays the primary role for determination of characteristics of hydrogen bond while a-skeleton only reflects die pi-polarization. An analysis of strength of intramolecular hydrogen bond using geometrical, energetic, and AIM and NBO parameters indicates that the homonuclear N center dot center dot center dot H-N hydrogen bond is considerably weaker than heteronuclear N center dot center dot center dot H-O and N center dot center dot center dot H-S hydrogen bonds in the case of the XH tautomers.