Water Supplies Department has been conducting a pilot desalination plant study since 2003 to explore the feasibility of using desalinated seawater as an alternative fresh water resource in Hong Kong. The pilot plant employs seawater reverse osmosis technology with a nominal capacity of 200 cu m/day. The objective of the study is to evaluate the technicalities by testing alternative pre-treatment methods and RO membranes, giving due consideration to production cost and reliability. Two sites, namely, Tuen Mun and Ap Lei Chau, are selected for the pilot testing so as to evaluate the impact on the operation of the plant due to seawater variations. The estuarial water quality at Tuen Mun is under the influence of Pearl River with high turbidity and low salinity whilst the water quality at Ap Lei Chau represents the oceanic waters with relatively low turbidity and high salinity. The presentation will describe the work performed during the study, with particular emphasis on the findings and experience gained from the operation and testing of the pilot plant at Tuen Mun.