Membrane fouling is a frequent problem in most seawater reverse osmosis units, particularly when raw water is drawn from an open sea intake. Membranes tend to foul slowly, which makes it necessary to perform periodic cleanings with physical and chemical products, which in turn produce a decrease in the useful life of the membranes. The specific objective of the research was to identify a rational chemical cleaning method and also to select suitable cleaning agents and their dosages. A number of experimental cleaning tests were carried out on a pilot plant with membranes previously used, and fouled, in a large seawater desalination plant (Las Palmas III). The procedure included performance tests before and after cleaning, and the cleaning itself, with acid and basic chemicals, as well as detergents. The results are presented in terms of four parameters to correlate the cleaning effect on the membranes: product water, conversion factor, salt rejection and turbidity. The cleaning agents that proved most effective for increasing product water flow rate were solutions such as basic chemical with detergent, acid with detergent, and also cleaning in two stages: hydrochloric acid plus caustic soda. Meanwhile the cleaning procedures that improved the salt rejection were in two stages: acid with detergent and basic with detergent.