The density maximum of water is probably its best known anomaly and attempts to account for this phenomenon go back to Roentgen in 1892 and possibly further. We would like to tackle this issue by a careful comparison between the momentum distribution and mean kinetic energy of both protons and deuterons of light and heavy liquid water. The evidence for quantum effects on the structure of water has been recently indicated, as well as a coherent delocalization of the proton between two neighbouring water oxygens. However, detailed Deep Inelastic Neutron Scattering measurements on heavy water are still lacking, and these are the object of this experiment.