Due to their excellent resistance to a range of solvents, integrally skinned polyimide membranes have been used to achieve selective separations in a range of solvent-based industrial and lab-scale chemical operations. These include: homogeneous catalyst recycle, petrochemical dewaxing, solvent exchange and chiral resolutions. However, despite the widening scope of use of these membranes, there is still little understanding of how the microstructure defines their separation performance. As a first step towards determining this, integrally skinned nanofiltration membranes were fabricated by phase inversion using Lenzing P84 polyimide. The microstructures of these membranes, dry and wetted in solvent, were investigated by SEM, TEM and ESEM (where appropriate). SEM and TEM imaging of dry membranes revealed that this type of polyimide membrane has three microstructurally distinct polyimide layers, not the two indicated in prior literature. Furthermore, TEM images reveal nano-sized pore-like features in the polyimide structure, which indicate that the transport mechanism is probably neither only solution-diffusion nor only pore flow. ESEM imaging showed that when saturated in ethanol at the working pressure of the ESEM (5.50 Torr), the microstructure of the membranes changes; it is wispy and thus quite different to the more solid polymer nodules and interconnected polymer network observed in the dry membranes. Thus, transport and separation mechanisms based on the structure of the dry membranes may not be accurate. Overall, these results indicate that the current theory used to describe polyimide membrane mass transfer and separation performance (where it is based on dry membrane microstructures), most likely needs to be rethought. (C) 2008 Elsevier B.V. All rights reserved.