6FDA-pMDA polyimide membranes were implanted with 140 keV N+ ions to fluences between 2 x 10(14) and 5 x 10(15) cm(-2). Variable energy positron annihilation spectra were taken and spectral features compared to previously reported changes in gas permeability and permselectivity of these membranes as a function of ion fluence. Positron data corroborate the explanation of these changes in terms of molecular damage caused by the implant for fluences up to about 1 x 10(15) cm(-2), the concentration of irradiation-induced defects merely increases with implant fluence; while fluences exceeding this threshold value create a second type of positron annihilation site, thereby marking a distinct change in the structure of the polymer, which is responsible for the vast improvement of gas permselectivity data found at the same threshold fluence. PACS codes : 78.70.Bj-positron annihilation; 61.82.Pv-polymers, organic compounds; 61.72.Ww-doping and impurity implantation.