Positron annihilation spectroscopy (PAS) has been performed to link fundamental polymer properties to membrane performance, more specifically for polyimide (PI)-based solvent-resistant nanofiltration membranes. Laboratory-made membranes with well-known properties were applied first to define proper pretreatment conditions for the membrane to allow PAS-analysis and to allow more correct linking of PAS results to membrane properties. This knowledge was then applied to probe the structure of commercial PI-based Starmem membranes.