Theoretical predictions and interpretations of highly excited vibrational states and their organisation are among the most important challenges in molecular spectroscopy. In this Letter we investigate the effects of isotopic mass substitution in the well-known normal-to-local vibrational modes bifurcation of the water molecule. Considering the mass of one hydrogen atom as a second continuation parameter we calculate energy-frequency continuation/ bifurcation diagrams of the principal families of periodic orbits. Extended quantum mechanical calculations reveal the localisation of the overtone eigenfunctions along the periodic orbits. The study of the changes of the phase space structures with mass variations unravel the fingerprints of bifurcations effects in the quantum states of isotopologues. (C) 2010 Elsevier B. V. All rights reserved.