The use of the Second Law of Thermodynamics to describe and quantify the exergy losses involved in the different processes taking place in desalination plants is of considerable importance both for the design and operation of these plants. Although the First Law of Thermodynamics is adequate in giving the overall plant performance indices, it does not show the actual irreversibilities in the different parts of the plant. An estimation of the exergy destruction involved in each part of the plant gives a quantitative measure of these irreversibilities which, if reduced during design or operation, can result in a reduction of energy consumption and increase plant performance. Based on actual measured data from a multiple-effect stack seawater desalination plant now in operation in the solar plant near Abu Dhabi, the exergy destruction was calculated for each source of irreversibility. The major exergy destruction was found to be caused by irreversibilities in the different pumps with the vacuum pump representing the main source of destruction. Major exergy losses are associated with the effluent streams of distillate, brine blow-down and seawater. Exergy destruction due to heat transfer and pressure drop in the different effects, in the preheaters and in the final condenser and in the flashing of the brine and distillate between the successive effects represents an important contribution to the total amount of exergy destruction in the evaporator.