Reverse osmosis/nanofiltration (RO/NF) membranes are capable to provide an effective barrier for a wide range of contaminants (including disinfection by-products precursors) in a single treatment step. However, solute rejection mechanisms by RO/NF membranes are not well understood. The lack of mechanistic information arises from experimental difficulties faced when evaluating water/solute transport phenomena within the ultrathin membrane active layers (less than 150 nm) of RO/NF membranes. The main objective of this study was to apply Rutherford backscattering spectrometry (RBS) to determine the partition coefficients of arsenious acid (H[subscript 3]AsO[subscript 3]) and other solutes, and the concentration of charged chemical groups in the active layers of RO/NF membranes with the goal of elucidating the mechanisms underlying the difference in H[subscript 3]AsO[subscript 3] rejection between commercial polyamide RO/NF membranes. Then, the role of water permeability, the H[subscript 3]AsO[subscript 3] partition coefficient, and the H[subscript 3]AsO[subscript 3] diffusion coefficient in H[subscript 3]AsO[subscript 3] removal efficiency was assessed to find key water/H[subscript 3]AsO[subscript 3] transport phenomena controlling H[subscript 3]AsO[subscript 3] removal efficiency. Experimental observations were then used to provide recommendations for physico-chemical properties of polyamide active layers that would result in high H[subscript 3]AsO[subscript 3] removal efficiency. Another main objective of this study was to investigate the influence of active layer hydrophilicity on solute removal efficiency. This objective has been achieved by characterizing Rhodamine-WT and H[subscript 3]AsO[subscript 3] removal efficiency by newly developed RO/NF membranes having rigid star amphiphiles (RSAs) as an active layer material. The knowledge obtained from this study will also be useful to guide the development of more effective RO/NF membranes. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]