When fresh water is produced from salt water or similar by evaporation of untreated or crude liquid in an evaporation device under partial vacuum and by vapor condensation in a condensation device connected with the vapor outlet of the evaporation device a high degree of evaporation and cost effectiveness can be obtained in that the evaporation device and the condensation device in a disconnected condition, are filled with crude or clean liquid, respectively, and are subsequently exposed to a partial vacuum created by volume enlargement under hermetically sealed conditions and that the evaporation device and the condensation device are not flow-connected with each other until they are under partial vacuum.