The performance of a membrane is a key factor in predicting its reliability. Designing membranes depends on their morphol. and mol. characteristics for specific applications. This work focuses on improving the behavior of a cellulose acetate-polysulfone PSf-CA membrane in the presence of PEG 400 additive, in different concentrations, using thermal annealing on the surface of the membrane. The membrane was prepared by the NIPS phase inversion process; the surfaces of the obtained membrane films were subjected to thermal annealing at 75 °C for 15 s. The influence of the composition of the polymer mixture, the concentration of the additive and the annealing effect on the performance of the membranes were studied. The membranes were characterized in terms of contact angle and mech. resistance, as well as by FTIR, DSC, ATG and SEM. Measurements of the contact angle confirmed the hydrophilic character, which increased with a rising level of PEG in the casting solution The results showed that the permeation flux increased for the highest PSf/PEG ratio. SEM results suggested that the annealing treatment improved the surface of the membrane; the surface layers became denser and smoother, with a porous intermediate layer, as CA concentrations rose in the mixture The fluoride reject was evaluated for all the membranes, the obtained fluoride retention rates were very satisfactory and met the standards required by WHO.