The isobaric heat capacity of liquid H2O, C-p, as a function of temperature, decreases between 0 degrees and about 35 degrees C and then increases up to 100 degrees C. Analogous behaviour is shown by liquid D2O. A statistical thermodynamic model has been applied to the experimental heat capacity data. The behaviour is explained by assuming that an equilibrium A + B = AB is established between clusters A and AB of water of different composition. The total heat capacity is considered as the sum of three terms C-p = (1-alpha)C-p,C-0,C-AB + alpha C-p,C-0,C-A + Delta C-p,C-app. The term Delta C-p,C-app depends explicitly on the reaction enthalpy. In H2O, the enthalpy Delta H = -1.84 kJ mol(-1) for the dissociation reaction and the heat capacity C-p,C-B = 47.8 J K-1 mol(-1) for free water molecules are calculated. Analogous calculations performed for D2O yield the enthalpy, Delta H = -1.64 kJ mol(-1) and the heat capacity, C-p,C-B = 49.18 J K-1 mol(-1).