A membrane restoration program was conducted at the site of a large (over 3 MGD) reverse osmosis brackish water desalting plant. The procedure applied consisted of cleaning the membranes with an alkaline solution at low pressure followed by dosing with a coloidal solution of high molecular weight polymer at high pressure. The initial restoration tests were performed on single 8″ elements, which were taken out from the commercial desalting unit due to the excessive salt passage. After obtaining positive results with a few single elements the restoration procedure was applied on the elements as installed in the desalting units. One of the units, equipped with 8″ and 10″ hollow fiber elements, had an average salt passage of 14%. Following the restoration it was reduced to a level of 6% to 10% and was maintained at that level for several thousand operating hours by applying continuous online treatment. The reduction of salt passage was accompanied by a decrease of about 10% in productivity. A similar treatment was applied to a number of old 8″ polyamide, spiral wound elements, which had a salt passage in the range of 14% to 24%, and were therefore disconnected from the unit. As a result of the treatment, an average decrease in salt passage of 40%, accompanied by a tolerable (≈10%) decrease of productivity, was obtained. The treated spiral wound elements were reinstalled in the desalting unit and operated for over a thousand hours with no significant increase of salt passage.