By utilizing the evolving nature of liquid water subjected to controlled heating, mid-infrared profiles acquired in the interval 2-96 degrees C are resolved into two spectra that can be interpreted as representing an equilibrium between two water structures differing in the average number of H bonds per water molecule. The structure with the lower number of H bonds is increasing with increasing temperature at the expense of the other. Assuming that the structures differ on the average of half a H bond per water molecule, the enthalpy and entropy change for the transformation from the stronger to the lesser H bonded structure were calculated as 2.2 kcal mol(-1) and 7.3 cal mol(-1) K-1. This is in excellent agreement with previously reported results for a transformation of water clusters consisting of;polycyclic octamers or decamers into clusters of monocyclic tetramers or pentamers.