Since 1992 Amsterdam Water Supply has been conducting pilot-plant (2 x 9 m(3)/h) research on the use of reverse osmosis (RO) for direct surface water treatment from the Rhine River. The investigations were made as a feasibility study for an extension with 13 Mm(3)/y of the existing treatment plant (70 Mm(3)/y). Research was carried out with ultralow-pressure membranes and an organophosphonate antiscalant (Permatreat 191). With this antiscalant no biofouling was introduced upon its use. Because of its low environmental impact, sulphuric acid was preferred above hydrochloric acid, but the barium sulfate scaling risk was increased. Experiments were conducted with both types of acid in increasing recovery steps from 80% up to 90%. While starting experimental difficulties with dosing the antiscalant occurred and BaSO4-scale formation was a fact, especially in the installation with sulphuric acid, a severe MTC decrease was seen. Successive cleaning was possible with an alkali cleaner containing a complex builder. From the barium mass balance over this 3-month period, the precipitated barium amount was calculated. Barium analysis of the cleaning solution showed an almost complete recovery of the barium from the membrane elements. Moreover, the MTC was restored completely. From this it was concluded that the precipitated barium sulphate was washed out completely. Recoveries up to 87% are possible without BaSO4-scale formation and run times up to 1 year. The working inhibition time of the antiscalant in concentrate from increasing recoveries was determined experimentally.