In the current work, the diagnosis of a reverse osmosis desalination unit is reported. Over the two last decades, the studied desalination unit was supplying a 1,200 bed hotel. The feed water was driven from a well near the sea. The desalination unit has two stages giving an average recovery equal to 81%. The behaviour of all water streams with respect to aggressiveness and scaling tendency was assessed. The second stage reject water was shown to exhibit a very high scaling behaviour with an instantaneous precipitation in the absence of feed water pH adjustment. The analyses have shown that the produced water was very aggressive. The second stage module autopsy has revealed a sharp decrease in the membrane performances because of mineral as well as organic fouling. The inorganic scale was essentially made of coesite, calcite and kaolinite clay. The presence of silica and clay was attributed to an inadequate pre-treatment process, whereas the presence of calcite crystals at the membrane surface reveals that the chemical inhibition performed at the pre-treatment process without adjusting the pH was not able to prevent calcium carbonate precipitation. A periodic acid wash of the second stage membranes is then necessary to guarantee the desired objectives of this stage.