Combination vibrations of neighbouring H-bonded molecules show a characteristic absorption band. This band appears in the spectra if H-bonds are formed and vanishes when the H-bonds are broken. Different observations allow the conclusion that the combinationvOH+vODin spectra of liquid HOD (in D2O) may be assigned to a combination vibration (vOH+vOD) 2molof two neighbouring molecules (HOD and D2O). The intensity of thisvOD) 2molband was investigated at different temperatures. It results an estimation of the van't Hoff enthalpy change of H-bond formation in liquid water ΔHo(OH O) = -15.4 ± 1.0 kJ/mole (= -3. 7 ± 0. 2 kcal/mole).