AbstractHigh free volume polymers are characterized by much higher permeability and diffusivity for gases and vapors than commodity polymers. Polymers of intrinsic microporosity (PIMs), an important member of this class of polymers, are only soluble in a few solvents. This gives reason to expect stability in many solvents and an application in organic solvent nanofiltration (OSN) may be feasible. Thin film composite membranes of PIM-1 and PIM copolymers were developed on a polyacrylonitrile (PAN) porous support. For control of swelling, a simple, technically realizable method of cross-linking was produced by blending the PIM with polyethyleneimine, coating to give thin film composites (TFCs) and thermally or chemically cross-linking the separation layer on the supporting membrane. The TFCs were tested in OSN with the solvents n-heptane, toluene, chloroform, tetrahydrofuran, and alcohols, and compared to similarly cross-linked poly(trimethylsilyl propyne) TFCs and state of the art industrial Starmem™ 240 membranes. Better retention, a steeper retention curve and much higher fluxes were detected for the newly developed PIM TFC membranes.