By semi-empirical quantum chemical method AMI it was first established that the diketoform is the main prototropic tautomer of isatin, the relative energy of enolic tautomers exceeding 16 kcal/mol. It is expected that exactly the diketoform of isatin must be observed in polar environment, since it has a significant dipole moment (5,1 D), which is distinctly higher than analogous values for most energy- favorable enolic tautomers (2,4 and 2,7 D). For the first time the acid-base properties of this amphoteric compound have been calculated in main tautomeric state. Imino group was shown to be the most acidic, while 'the oxygen atom of adjacent carbonyl gruop – the most basic, the associated proton acguiring trans- orientation against the adjacent imino group. As compared with nucleotide bases, isatin possesses much weaker properties both as proton acceptor and proton donor.