The membrane, a critical component of a vanadium redox flow battery (VRFB) cell, largely governs the battery performance through its barrier properties and ohmic resistance, as well as contributes substantially to cell stack cost in commercial systems. Polybenzimidazole (PBI) is emerging as a promising commercially available material for VRFB membranes with properties that can be tailored through suitable low-cost pretreatments. Herein, we demonstrate how pre-swelling in blended alkaline solutions can be a highly effective and accessible strategy to improve PBI conductivity for application as a VRFB membrane, whilst enabling the tunable balancing of trade–off relations with electrolyte crossover that lead to undesirable capacity fade. A figure of merit is determined, showing that the inclusion of fractions of both NaOH and KOH in the pretreatment mixture is deemed necessary to ensure that both vanadium permeability and area-specific resistance are sufficiently minimized, respectively, in the membrane by achieving optimal, intermediate levels of swelling. By applying blended pretreatments, PBI membranes could be tailored to result in high performance cell tests, exhibiting stable and high energy efficiencies of ∼85 % with minimal capacity fade over 100 cycles at 120 mA·cm⁻².