Recently, single-stage seawater reverse osmosis (SWRO) in the Middle East has been developed. In this process, removing dissolved oxygen (DO) from the feed seawater is very important for membrane life. The reaction rate of sodium bisulfite (SBS) and DO in seawater was investigated to apply DO removal system to the SWRO. The reaction in seawater was very fast because the salinity had a positive catalytic effect and the bicarbonate ion kept the pH suitable for the reaction. By applying these results, Toray's PEC-1000 membrane modules have shown excellent performance and long durability.