The post-Hartree-Fock ab initio studies were performed to predict the properties of eight tautomers of 5-azacytosine. All geometries were optimized without symmetry restrictions by the gradient procedure at the MP2 level of theory using the standard 6-31G(d,p) basis set. Single-point calculations have been performed at the MP2/6-311++G(d,p), MP2/6-31 ++G(2df,2pd). MP4(SDTQ)6-31G(d,p), and CCSD(T)/6-31G(d,p) levels for all eight tautomers. The calculations of the stabilities of the tautomers in a polar medium have been performed using explicitly water molecules which form a first solvation shell, and PCM solvation model. The results are compared to the corresponding data for cytosine tautomers. Ab initio calculations predict a different order of tautomers in cytosine and 5-azacytosine; however, the relative stabilities of the two lowest energy tautomers are the same for both bases. The molecular geometries of guanine-5-azacytosine and guanine-cytosine base pairs have been optimized using the MP2/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. The interaction energies have been calculated at the MP2/6-31G(d,p), MP2/6-31 1++G(d,p), MP4(SDQ)/6-31G(d,p), and B3LYP/6-31G(d,p) levels and corrected for the basis set superposition error. The interaction and solvation energies of base pair complexes with water have been estimated using first solvation shell of water molecules and PCM solvation model. The study has shown the similar geometrical parameters for fragments of both bases associated with the formation of hydrogen bonds with guanine and different molecular parameters associated with the moieties involved in the interactions with cytosine-5-methyltransferase.