Wyss and Falk's infrared absorptivities between 10 and 85 °C for HDO in liquid D2O in the fundamental O—H stretch region have been analyzed numerically and show a statistically significant weak band near 3600 cm−1 at each temperature. This band, the Raman analog of which was reported earlier, confirms the presence of HDO species with unbonded O—H groups in the liquid state. The percentage of HDO species with un-bonded O—H groups is found to be 4.6, 6.1, 10.1, and 11.9, respectively, at 10, 35, 60, and 85 °C.The four predominant HDO species in equilibrium at low temperatures are denned together with their equilibrium constant for the [Formula: see text] bond-rupturing process. A standard enthalpy change of ΔH° = 6800 ± 1100 cal is found for the rupture of one mole of [Formula: see text] bonds. This is in contrast with the apparent enthalpy change of 2900 ± 500 cal, as usually derived from the temperature dependence of the equilibrium quotient. Approximate mole fractions for each of twelve differently bonded HDO species are derived from a model for random H- and D-bond statistics. Results show that the four predominant HDO species give a full interpretation of the absorptivities at 10, 35, and 60 °C but that additional minor species contribute slightly to the absorptivity at 85 °C.