Abstract: In order to treat water contaminated with warfare agents and containing less than 500 ppm of salt, a separation stage 10 is normally provided downstream of the reverse osmosis. In the separation stage, the untreated water is divided into residue and permeate. Warfare agents have the property of acting as softeners on the semi-permeable organic polymer membranes of separation stages. As a result of this, the warfare agents pass through the membranes of separation stages in increasing quantities once the latter have been in operation for a few hours. In order to improve the treatment of water contaminated with warfare agents, salt is added to the untreated water which, with a salt content of over 1000 ppm, is then drawn through the separation stage 10 down stream of the reverse osmosis stage. The invention is suitable for use with mobile facilities used for producing drinking water.