Triple-resonance vibrational spectroscopy is used to determine the lowest dissociation energy, D0, for the water isotopologue HD16O as 41 239.7 ± 0.2 cm−1 and to improve D0 for H216O to 41 145.92 ± 0.12 cm−1. Ab initio calculations including systematic basis set and electron correlation convergence studies, relativistic and Lamb shift effects as well as corrections beyond the Born–Oppenheimer approximation, agree with the measured values to 1 and 2 cm−1 respectively. The improved treatment of high-order correlation terms is key to this high theoretical accuracy. Predicted values for D0 for the other five major water isotopologues are expected to be correct within 1 cm−1.