In the present work, the influence of the lithium bromide (LiBr) additive on the performance of polyetherSulfone (PES) membranes was studied using various formulations of dope solutions to which the microwave assistance (MWA) method was applied. The amount of PES was kept at 20 wt %, and the weight ratios of LiBr to dimethylformamide (DMF) were varied in the range 1-5 wt %. Results revealed that MWA irradiation increases the kinetics of dissolution of PES even in the presence of higher ratios of LiBr. It was found from viscosity, FTIR, water uptake, contact angle, and SEM-EDX analyses that the interaction between PES and LiBr is strongest when their mole ratio is unity. Performance data and the pore size distributions on the membrane surface have also been investigated. The structure of the MW casting solution under a strong interaction force attributed to the irradiation causes a decrease in the pore sizes and results in an increased rejection rate through the ultrafiltration membrane.