Continuous research and development of reverse osmosis (RO) technologies has led to the production of membranes that are very effective with high salt rejection abilities. As temperature is one of the factors that affects salt rejection capabilities in membranes, this paper investigates the effect of temperature on the thickness of the concentration polarization layer (CPL) deposited on thin-film composite seawater RO membranes. Two types of membranes were studied: those with ex situ macromolecules and those with in situ macromolecules. FilmTec's reverse osmosis system analysis design software was used to predict the variation of salt rejection and permeate flow rate with temperature. The impact of these variations on the thickness of the CPL was analyzed for different polyamide concentrations in the membrane.