The quality and quantity of reverse osmosis (RO) permeate may vary, depending on the applied pressure, temperature, conversion factor and the feed concentration. Actual performances are not steady under various conditions, and thus operating data are generated variably. An effective evaluation of the performance of the RO system is to compare permeate flow rate and salt rejection data under standard conditions. At the Doha desalination plant (DRP) in Kuwait, an on-line standardization software program, based on the ASTM I method, has been developed to evaluate the performance data of the two RO lines - spiral wound and hollow fine fiber. This paper investigates the performance of the two RO lines using the program developed and comparing the results with the design data as a reference. For this purpose operating data, generated over two years by DRP, were utilized.