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Abstract Non‐aqueous redox flow batteries can support the growing need for grid scale energy storage. Conductive and selective membranes are critical to enabling advanced flow battery technology with higher energy density and lower cost. Herein, a series of crosslinked poly(phenylene oxide)‐based membranes functionalized with phenoxyaniline tri‐sulfonate fixed charges are developed and reported. Increasing the degree of functionalization increased the theoretical ion exchange capacity (IEC) to 2.62 mEq g⁻¹compared with a previously achieved maximum of 2.06 mEq g⁻¹for these materials. Increasing membrane IEC increased conductivity (up to 0.41 mS cm⁻¹) and decreased permeability (<1 × 10⁻⁹cm²s⁻¹) based on ex situ measurements. While further improvements are still necessary for economically competitive non‐aqueous flow battery applications, these values are among the highest selectivity reported for cation exchange membranes in non‐aqueous electrolytes. In symmetric non‐aqueous electrochemical flow cell testing, current densities over 2 mA cm⁻²are achieved, although cell polarization is higher than expected. Long‐term cycling is performed at 1 mA cm⁻²with average coulombic efficiency over 99% maintained for over 70 cycles. This work reinforces that both ex situ transport properties and electrochemical cell evaluation are necessary when evaluating potential membrane materials, although reporting of both is presently uncommon in the literature.