Abstract: Desalination apparatus using solar energy comprises a main insulated container with a subsidiary preheating container next to it; the main container is covered with a transparent cover serving as a selective filter to provide "the greenhouse effect" and also as a condensation surface for water vapor, and this transparent cover incorporates a hollow space, for example by making the cover double skinned. The hollow space forms part of a syphon which automatically regulates the water levels, transferring salt water from a preheating container to the main container as the level in the main container falls due to loss by evaporation leading to condensation and withdrawal of the desalinated water. The hollow part of the transparent cover thus serves also to preheat the incoming salt water with the heat lost upon condensation by the vapor within the main container.