A desalinator is disclosed wherein the energy required for evaporation is provided primarily by the energy released during condensation in a counterflow heat exchanger consisting of an outer chamber and an inner tube or tubes. Sea water is evaporated into air at ambient pressure in the inner tube; this air-vapor mixture is then heated and reintroduced into the outer chamber where it heats the contents in the inner tube as it cools and its vapor condenses to distilled water.