Equilibrium and transient sorption isotherms were obtained for toluene and n-heptane in both annealed and non-annealed Matrimid (R) asymmetric hollow fiber membranes at 35 degrees C. Equilibrium sorption follows the dual mode model except for toluene sorption into annealed fibers above a pressure of 0.5 psia. Except at the highest toluene exposure pressure, the equilibrium uptake of penetrant in annealed fibers was significantly less than in non-annealed fibers at a given pressure due to the significant reduction of excess free volume. Changes in the dual mode model parameters for the annealed samples may reflect not only reductions in sample free volume, but also charge transfer formation. The Berens-Hopfenberg model successfully describes all of the various transient sorption behaviors observed for toluene and n-heptane in Matrimid (R) with a significant relaxation-controlled contribution to the overall mass uptake over much of the experimental range explored. Purely diffusion-controlled (Fickian) uptake was seen only for toluene sorption in annealed fibers for a change in activity from 0 to 0.05, while purely relaxation-controlled (non-Fickian) uptake was observed for n-heptane sorption in non-annealed fibers for a change in activity from 0 to 0.09. A reduced value of D-A/L-2, the effective Fickian diffusion time, in non-annealed fibers for toluene at a low activity level provides evidence of antiplasticization, while at intermediate to high activity, the value of D-A/L-2 increases with activity for all of the systems studied in this work indicating plasticization. (C) 2009 Elsevier Ltd. All rights reserved.