This research explores the efficacy of quaternary ammonium-functionalized polymeric membranes in enhancing gas separation performance under different humidity levels. We synthesized brominated polyphenylene oxide and used it to prepare selective layers of thin film composite membranes. After coating, we implemented an in situ quaternization process for the brominated layer, followed by the subsequent exchange of bromine to chlorine counter ions. The spectroscopic characterization (FTIR and EDX) confirmed the inclusion of quaternary ammonium groups and the counter ion exchange. Under dry conditions, the quaternization followed by ion exchange significantly improved the CO2 /CH4 selectivity to 30 while decreasing permeance. Under 90 % relative humidity, the membranes achieved 68 Barrer CO2 permeability and a CO2/CH4 selectivity of 178, compared to 56 Barrer and a selectivity of 11 for the brominated (non-quaternized) polymer membranes. The subsequent exchange of the counter ions, specifically replacing bromine ions by chlorine, markedly enhanced the performance of the membrane. This pivotal modification promotes the CO2 permeance and augments its CO2 selectivity over methane when evaluated in highly humid and challenging environments.