Deuterium oxide (D2O) is a deuterated solvent. It is 100% isotopically enriched solvent for NMR (Nuclear Magnetic Resonance). It is widely employed in high resolution NMR studies. Various thermodynamic properties (such as intermolecular vibrational frequencies, energy of the hydrogen bond, free energy, enthalpy and entropy) of liquid deuterium oxide have been evaluated. Ionization constant for D2O (in the range of 5-50°C), pK values (at 25°C) and enthalpy, entropy, heat capacity change (for the dissociation of D2O) have been reported.