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Ion-exchange membranes are widely used in technologies for desalination and blue-energy harvesting. Their performance is usually measured in laboratory cells, with the assumption that the electrical response directly reflects the intrinsic properties of the membrane. In this work, we show that the geometry of the experimental setup itself can strongly influence the measurement. When a small membrane is connected to large reservoirs and electrodes, electric currents must “converge” toward the membrane, creating additional resistances that are not part of the membrane itself. By designing experiments where we could systematically vary the membrane size, we revealed how these hidden access effects can dominate the measured signal. This work highlights how subtle geometrical constraints can shape transport in electrochemical systems, and reminds us that measuring transport is often as challenging—and as interesting—as understanding it.