The Dhekelia Desalination Plant at Larnaca, Cyprus, includes eight 5,000m(3)/d seawater reverse osmosis desalination units operating with Mediterranean seawater with a TDS content of 41,500ppm from an open sea-intake at a water temperature range of 17 to 32 degreesC. The plant has been in operation for forty six months. Due to the difficulties encountered during the first months of operation many trial run tests and pilot tests have been carried out. This paper outlines the operating experience with different pre-treatment chemicals, filter media types, and cleaning chemicals, describes their effect on the performance of the seawater RO membranes and compares fouling incidence. This study also describes the experience encountered on different operating feed SDI's and why it has been decided to operate at higher SDI's/less pre-treatment chemicals vs. lower SDI's/higher pre-treatment chemicals and effect on performance and operational costs.