The paper is devoted to the experimental study of the leak-tightness of water-containing melt inclusions heated in the autoclave at high temperature and under the pressure of D2O. Quartz-hosted silicate-melt inclusions from the rhyolite tuffs of the Taupo volcanic zone (New Zealand) and a tourmaline-pegmatite vein from the SW Pamirs were heated at 650 °C and 3 kbar. The penetration of heavy water into the inclusions was controlled by IR spectroscopy. The studies have demonstrated that the inclusions can remain leak-tight under these conditions and not exchange water with the environment even if the confining pressure is 1.5–3 times above their internal pressure. The influence of water diffusion through the quartz lattice and dislocations on the leak-tightness can be neglected in the thermometry of water-containing melt inclusions. The crucial factors determining the water exchange between the inclusions and the environment in the experiments performed are mechanical defects (open and healed cracks). Using D2O as a pressurizing medium makes it possible to control the leak-tightness of the heated inclusions. To do this, it is recommended that melt inclusions be homogenized under the pressure of D2O above the expected entrapment pressure and studied by IR spectroscopy after the heating.