A combined ab initio+nuclear dynamics study is performed to theoretically analyze the intramolecular H-atom transfer process in 5-aminotropolone in both the ground (S-0) and first excited (S-1) singlet electronic states. A complete active space self-consistent field (CASSCF) method is used to optimize the geometries. Energies are then corrected through the second order Moller-Plesset perturbation theory. These results are used to build up reduced bidimensional energy surfaces so that the nuclear wave functions for the nuclear motions in both electronic states are obtained. In particular we have analyzed the six isotopomers that result from deuteration of the amino and hydroxy groups of 5-aminotropolone, It is found that for symmetric structures (-OH/-NH2, -OH-ND2, -OD/-NH2, and -OD/-ND2), the two lowest vibrational levels in both So and SI appear as a quasidegenerated tunneling doublet. The tunneling splitting in S-0 is much lower so that the doublet at the origin, seen in the fluorescence excitation spectra of 5-aminotropolone, can be entirely assigned to the S-1 state. In agreement with the experimental findings, this splitting greatly diminishes when the transferring hydrogen is substituted by a deuterium, whereas deuteration of the amino group produces only a modest decrease of such a splitting. A quite different result is found for the nonsymmetric isotopically substituted structures (-OH/-NHD and -OD/-NHD), as the isotope induced asymmetry, combined with the high energy barrier in the S-0 potential energy surface, leads to a complete localization of the two lowest vibrational wave functions in S-0. On the other hand, for S-1 the asymmetry and energy barriers are low enough so that an important degree of delocalization of the two lowest vibrational wave functions is found. These results are again in agreement with the presence of an isotope induced quarter in the fluorescence excitation spectra of these species.