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The molecular sieving behavior of soft‐etched polyimide membranes having negatively charged nanochannels is described experimentally and theoretically using alkali metal–crown ether cationic complexes and alkylammonium cations. To this end, the electrical conduction and current rectification obtained with different alkali electrolyte solutions (LiCl, NaCl, and KCl) and crown ether molecules (12‐crown‐4, 15‐crown‐5, and 18‐crown‐6) are studied. The results suggest that only the [Li(12C4)]⁺ complex can readily permeate through the nanochannels because significant current decreases are obtained in the cases of the [Na(15C5)]⁺ and [K(18C6)]⁺ complexes. In solutions of organic cations ranging from ammonium (NH₄⁺) to alkylammonium (R₄N⁺) with increasing molecular size, only the smaller ions can conduct high electric currents, suggesting again that the membrane channels are in the nanometer range. Taken together, the observed current decreases and rectification phenomena demonstrate that the functionalized membranes allow a versatile combination of molecular and electrostatic sieving.