Gas separation membranes can be physically or chemically modified to improve their separation characteristics. The ability of bromine to form charge-transfer complexes with unpaired electrons along a polymer backbone was utilized to tailor the gas separation properties of a polysulfone membrane. It was hypothesized that the bromine molecules would alter the free volume distribution in such a way as to hinder the transport of large gas molecules without a significant reduction in the flux of the more permeable gas. Bromine was sorbed into the polymer from an aqueous solution at pH 2. The treated membranes developed a characteristic orange color, indicative of charge-transfer complex formation, which remained after aqueous rinsing and vacuum drying. Carbon dioxide (CO2) and methane (CH4) permeability coefficients of the treated membranes were measured to determine the selectivity. After bromine treatment, the selectivity of a polysulfone membrane for CO2 over CH4 was increased over 100% at 10 atm upstream pressure with only a 36% reduction in CO2 permeability. Membrane stability was inconsistent with only one film appearing visually stable up to one year after treatment.