A new method of investigating tautomeric equilibria is described. The method is based on the partition of the total molecular electronic energies among atoms, according to the quantum theory of the atoms in molecules. We employ our methodology to study several model tautomeric systems, so called tautomeric triads. The differences in the atomic energies between tautomers are exhaustively discussed and general trends in relative stabilities of C, S, N and O atoms calculated in different molecular environments are presented.