Unlike the case for low-sorbing gas molecules such as N-2 and O-2, Matrimid (R) hollow fibers swell significantly in acetic acid (HAc) and water mixtures. Acetic acid was found to plasticize the polymer and facilitate the passage of the 'slow' molecule, acetic acid, while not affecting the 'fast' molecule, water, as significantly. The membrane selectivity of water over HAc inevitably decreased with increasing HAc content in the feed for the standard membrane. Sub-Tg (glass transition temperature) thermal annealing was used to stabilize large bore size defect-free Matrimid (R) hollow fibers by suppressing the HAc-induced plasticization. The resulting membrane selectivity was increased up to 95 using a 520 mu m outer diameter/325 mu m inner diameter and 20 cm long defect-free annealed fiber with acceptable water flux (1.6 kg/m(2) h) for 20 wt% HAc concentration feed streams at 101.5 degrees C. Micro-fluorescence spectra provide strong support for the notion that thermal annealing can reduce the free volume and promote the formation of charge transfer complexes. As a consequence, the improved rigidity of the polymer chains increases the polymer discrimination of shape and size for penetrants, although no chemical reaction occurs with thermal annealing. (c) 2005 Elsevier Ltd. All rights reserved.