Abstract Seawater flocculation was studied in this investigation as an aid in preventing fouling of reverse osmosis (RO) membranes by particulates. The behavior of clays in seawater and their possible use in the pretreatment process was also investigated. Mediterranean seawater samples were found to have a salinity of about 36‰, pH of 7.9–8.2 and turbidity of 1.0–17.5 NTU (Nephelometric Turbidity Units). After 1–3h of quiescent setting, 30–50% of the turbidity remained and then stabilized, pointing to a possible advantage of lagooning prior to further treatment. Tests carried out with artificial seawater, as well as natural seawater, were carried out to study particle removal in general, and algae and clay in particular. Different conventional and polyelectrolytic flocculants were used in the experiments. The best clarification was found using alum and an anionic polymer in combination, resulting in 20–100 particles per ml and a turbidity of less than 1 NTU. It was also found that particle size distribution measurements may provide a better picture of the quality and mechanism of treatment.