Triple-resonance vibrational spectroscopy is used to determine the lowest dissociation energy, Do, for the water isotopologue (HDO)-O-16 as 41239.7 +/- 0.2 cm(-1) and to improve D-0 for (H2O)-O-16 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 D-0 for the other five major water isotopologues are expected to be correct within 1 cm(-1). (C) 2013 Elsevier B.V. All rights reserved.