The results of pilot testing of dual media filtration on seawater from an open intake in the Thermaikos Gulf, northern Greece, are presented in this study. Two different filter materials, FILTRALITE MC, a processed expanded clay material with particle range 1.5-2.5 mm, and anthracite coal with a similar particle range, were tested in parallel operation under various operating conditions (coagulant type and dose, filtration rate). Attention was focused on filtrate quality parameters relevant to feeding RO membrane desalination systems, that is, turbidity, silt density index (SDI), total organic carbon (TOC). In order to evaluate the effect of temperature variations on pretreatment performance, similar experiments were performed during a winter and a summer campaign of the same year. Despite the poor quality of feedwater, through an open intake, very low turbidities and SDI values lower than 5 could be obtained by means of coagulation and dual media filtration. Both filtration materials demonstrated almost similar performance in removing particulates from the feed water and produced permeates of acceptable quality for feeding RO systems (SDI15 values lower than 5) during the winter and summer campaigns. It was observed that both filters were much more effective during the summer period due to the higher temperatures prevailing, which allowed the formation of larger aggregates and their subsequent effective capture within the filter media. Laboratory and on site pilot tests showed that optimal performance was achieved when polyaluminum chloride (PACT) was added to the feed water as a coagulant medium at a dose 1.8 mg Al/L. In the winter period similar SDI15 values were obtained by both filters when operating at a flow velocity of 10 m/h, whereas at 5 m/h the sand/filtralite column produced water with lower SDI15 values and better quality compared to the one from the sand/anthracite column. On the other hand, during the summer period, and for the same coagulant dose, similar SDI,5 values (usually close to 3) were obtained by both filters at all the tested flow velocities (5, 10, 15 m/h). Furthermore, due to the effect of temperature on aggregation, in the winter period the duration of filtration cycles was found to be determined by particle breakthrough. In contrast, in the summer no breakthrough was observed and the filtration cycles were determined by the available free space for hydrostatic head buildup above the filter media. Finally, regarding the hydrostatic head development, it was observed that the sand/filtralite column had a slower head buildup than the sand/anthracite column for the same duration of filter operation.