Abstract: Asymmetric integrally-skinned PAEK nanofiltration membranes suitable for nanofiltration of an organic solvent feed stream are disclosed, together with their uses in organic solvent nanofiltration, and their methods of preparation. Membranes are prepared from phase inversion processes and are then subjected to a post-manufacturing heat treatment step in order to arrive at molecular weight cut off characteristics within the nanofiltration region. The membranes exhibit stability over a wide range of p H and temperature.