The Al-Jubail SWRO plant has a unique feature of using two entirely different types of RO membranes. The plant has 15 trains, producing 20 MIGD product water of WHO recommended quality. Eleven of these trains use polyamide hollow fine fiber (HFF) membranes and the remaining four utilize polyamide spiral-wound (SW) membranes. Initially, the HFF permeators were installed in trains A, 13, C and D. Since the production of these permeators has been discontinued, an alternate solution had to be found. Hence the SW membranes were selected. The SW vessels have been arranged in such a way that three vessels are side-port connected in series. The vessels were specially fabricated so that the existing racks, feed/brine/product headers and manifolds could be utilized as they were already fabricated and installed. This study compared the performance parameters like salt rejection, feed pressure, and flux decline of these two types of membranes. The seawater temperature in the Arabian Gulf varies from 15 degrees C to 37 degrees C and the TDS from 42,000 mg/L to 45,000 mg/L. Among 15 trains (11 HFF + 4 SW), one train was selected from the SW group and another train from the HFF group for comparison purposes. The performance of the membranes changed with a wide fluctuation of feed water temperature. As per the plant design, the product water flow rate remains constant in spite of variations in feed water temperature. The software at the DCS reduces the feed pressure if temperature increases or vice versa.