Polysulfone hollow fiber membranes with increased free volumes as reflected by elevated glass transition temperatures can be obtained from a spectrum of Lewis acid : N-methylpyrrolidone (NMP) complexes provided that the complex is readily dissociated by water. The upper limit of the candidate Lewis acid for suitable complexes is an acceptor number between 49.1 (propionic acid) and 52.9 (acetic acid). Although the enhanced free volume yields an increase permeability with maintenance of selectivity, a knowledge of the structure and morphology of both the active separating layer and the substructure is needed to understand the performance of the membrane. Oxygen plasma ablation experiments have shown that the active separating layer of these membranes decreases in the order NMP/FA > acetic acid : NMP > propionic acid : NMP, butyric acid : NMP, and isobutyric acid : NMP with the differences among the last three being too small to resolve by the current experiment. Also, the surface and internal pore size and internal porosity appeared to increase in the order NMP/FA < acetic acid : NMP < propionic acid : NMP < isobutyric acid : NMP < butyric acid : NMP.