Heat of vaporization of water slightly decreases with temperature. https://en.wikipedia.org/wiki/Enthalpy_of_vaporization Does anybody know of a simple explanation of this fact based on molecular theory? Or any explanation whatsoever. The Wikipedia article says " The heat of vaporization diminishes with increasing temperature and it vanishes completely at the critical temperature (Tr=1) because above the critical temperature the liquid and vapor phases no longer co-exist. " but this does not explain why the heat decreases.