Abstract: Method and apparatus for desalinizing water. Salt water in an evaporation zone is provided with heat of vaporization by a heat pump which transfers heat to the water from a thermal sink. The evaporation zone is maintained at subatmospheric pressure so that the salt water boils at a temperature at least about 10 degrees above the ambient dew point. The pressure in the evaporation zone is maintained at a low level by a vacuum pump and by condensing the vapor in a condensation zone from which fresh water is collected. The heat of condensation is dissipated into a "cold" sink if available or by a heat dissipating means such as an evaporative cooling apparatus. Energy efficiency results from the ability of the heat pump or pumps in the system to operate at a high coefficient of performance.