Organic Solvent Nanofiltration (OSN) is a type of membrane process with increasing use in the pharmaceutical industry for the separation of small molecules.[1] The membrane process allows for the concentration of crystalline mother liquors while recovering solvent which can be used within the same step in a later batch. There are many different types of membranes available, but polymer spiral wound and flat sheet modules are most often used for OSN due to its stability, resistance to harsh chemical environments, low fouling tendency and performance. Selecting an appropriate membrane is a complex process, as there are many considerations involved, including the pore size, the compatibility of the solvent with the properties of the material and the desired flux from the recovery process.