The impact of conditioning on the permeability and permselectivity of CO2 and CH4 is discussed for two polyimide isomers that have been conditioned with carbon dioxide using several conditioning protocols. Partial depressurization conditioning at pressures that would induce approximately 8% volume dilation; resulted in up to 50% increases in the permeability, diffusivity, and solubility coefficients of the polyimide isomers. The para-connected isomer, GFDA-6FpDA, exhibited a greater conditioning effect following partial depressurization conditioning with CO2 at 8% volume dilation than did the meta-connected isomer, 6FDA-6FmDA. Conditioning of 6FDA-6FpDA followed by an exchange with an equimolar mixture of CO2 and CH4 resulted in an increase in the methane permeability and a slight decrease in the permselectivity relative to the film prior to conditioning.