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Water filtration membranes with advanced ion selectivity are urgently needed for resource recovery and the production of clean drinking water. This work investigates the separation capabilities of cross-linked zwitterionic copolymer membranes, a self-assembled membrane system featuring subnanometer zwitterionic nanochannels. We demonstrate that selective zwitterion–anion interactions simultaneously control salt partitioning and diffusivity, with the permeabilities of NaClO 4 , NaI, NaBr, NaCl, NaF, and Na 2 SO 4 spanning roughly three orders of magnitude over a wide range of feed concentrations. We model salt flux using a one-dimensional transport model based on the Maxwell–Stefan equations and show that diffusion is the dominant mode of transport for 1:1 sodium salts. Differences in zwitterion–Cl − and zwitterion–F − interactions granted these membranes with the ultrahigh Cl − /F − permselectivity ( P Cl- /P F- = 24), enabling high fluoride retention and high chloride passage even from saline mixtures of NaCl and NaF.