The results of an ab initio post Hartree−Fock study of the molecular structures, relative stabilities, and mechanisms of intermolecular proton transfer in isolated, mono- and dihydrated guanine complexes are reported. The geometries of the local minima and transition states were optimized without symmetry restrictions by the gradient procedure at the HF and the MP2 levels of theory and were verified by energy second derivative calculations. The standard 6-31G(d) basis set was used. The single point calculations have been performed at the MP4(SDQ)/6-31G(d)//MP2/6-31G(d) and the MP2/6-311++G(d,p)//MP2/6-31G(d) approximations. All values of total energies have been corrected for zero point energy contributions scaled by a factor of 0.9. The post Hartree−Fock ab initio theory predicts the height of the proton transfer barrier for monohydrated guanine complexes to be approximately two times lower for the tautomeric oxo−hydroxo reactions and approximately three times lower for the reverse hydroxo−oxo reactions compared with non-water-assisted processes. The influence of polar media (Onsager's self-consistent reaction field model) slightly changes these values according to the polarity of the tautomers. The influence of the stepwise interaction with one and two water molecules monotonically changes the order of the relative stability of guanine tautomers from the gas phase to the one which corresponds to the experimentally measured relative stabilities in polar solutions. We have found a 2-fold water influence on the NH2-nonplanarity phenomena:  they are the source of nonplanarity for the hydroxo tautomers, and they also decrease the nonplanarity for the oxo tautomers.