In the manufacture of heat-exchangers, it is of great importance that the heat-exchanger plates be thoroughly cleaned before being sealed. At a Swedish heat-exchanger manufacturer, oil, grease and other impurities are removed in a washing plant consisting of three stages: a degreasing tank and two rinsing tanks. The possibility of improving the cleanliness of the heat-exchanger plates, without increasing the need for fresh water, by treating the rinsing water in a reverse osmosis (RO) plant, was studied in this investigation. A new, high-temperature-resistant, thin-film RO membrane from Osmonics-Desal was used in the experiments. The influence of a lubricant, based on a block copolymer, and an alkaline degreasing agent on the membrane performance was studied. The flux reduction of a model rinsing solution with time was moderate. The flux after 500 h of continuous treatment at 65 degrees C and 4.0 MPa was 351/m(2) h, and the permeate quality fulfilled the demands of the heat-exchanger manufacturer.