The prototropic molecular-zwitterion tautomerism of hypoxanthine (Hyp) was investigated by means of semiempirical quantum mechanical AMI method with full parameters optimization in vacuum. It was established that rnolecular-zwitterion family of Xan occupies the energetical range about 26 ccal/mol and it consists of 12 structural isomers – 10 molecular and 2 zwitterion; all they are plant dipole-stable structures. According to calculation, at the room temperature in the gase phase there are two energetically prevalent molecular ketone tautomers of Hyp–N9H and N7H. More polar form N9H is the main (82 %). At the same condition concentration of enol tautomers is less than 0.2%. These results are quantitative conformation of tautomeric equilibrium of Hyp in vacuum, which was investigate from IR spectra of low temperature matrix isolation (Sheina G. G. et ah, 1986). Under transition to solvent with universal solvation mechanism the equilibrium shifts some more to the N9H tautomer (98.6 %); concentrations of all other tautomers except NTH (1.4 %) are very small. It was established, that the main ilid form (which has iminoproton at Nl, N7 and N9 atoms and has no C8H proton) is an intermediate state of hydrogen-deutero-tritium exchange of C8H group in vacuum. The calculated value of energetical barier of this reaction is in a good agreement with the experimental data, and, to our mind, this agree confirms the ilid mechanism of hydrogen-deutero-tritium exchange of C8H group in the water under the acidic and neutral pH In the frame of results obtained the reasons of «substitution» of guanine to Hyp in an-ticodon triplets of tRNA are discussed.