The time-dependent permeation behavior of a glassy polyimide is studied above and below the plasticization pressure with carbon dioxide as the permeating gas. The work particularly focuses on the quantification of the slow increase in permeability at feed pressures above the plasticization pressure. Significant differences in permeation behavior were found for membranes with and without permeation history. Introducing the quantity conditioning time allows one to compare the results obtained for membranes with different permeation history. The concept of conditioning time furthermore allows one to quantify the aging of a membrane plasticized during previous permeation.