Physical aging in high free-volume polymer membranes is one of the main hurdles limiting their application in gas separation. The recovery of membrane separation properties without the need to disassemble the module, although challenging, would provide significant advantages for applications in various fields. In this work, an in situ restoring procedure for the recovery of mass transport in aged membrane modules made of thermally rearranged polymer membranes was developed in which the modules were exposed to methanol at 80 degrees C. The thermally rearranged hollow fiber membrane modules were subjected to long-time operation to investigate their aging: the CO2 and N-2 permeances were monitored at different temperatures, pressures, and feed compositions over a total period of 727 days with two long-time runs of 185 and 28 days, interspersed by a stand-by period of 240 days, and with each run followed by a restoring. In both long-time runs, CO2 and N-2 permeance dropped as a result of aging, whereas the selectivity remained nearly constant. The permeances were fully recovered after the proposed restoring procedure was applied, demonstrating its efficacy and repeatability for membrane aging recovery, even for an extremely aged membrane exposed to various conditions for nearly two years. (C) 2016 Elsevier B.V. All rights reserved.