PIM-1, PIM-7, and PIM-8 composite membranes have been fabricated for Organic Solvent Nanofiltration (OSN) on two different support membranes. Both support membranes, PAN and crosslinked Ultem 1000, displayed pore sizes within the range of 20–25 nm as characterised by gas liquid porometry. PIM layers of < 500 nm thickness were formed from dip coating on a roll-to-roll pilot line. The resultant composite membranes exhibited typical MWCOs in the region of 500–800 g mol−1. The quality of coating obtained on the crosslinked Ultem 1000 support membrane was consistently higher for all three PIMs than that obtained on the PAN membrane. The PIM composite membranes coated on to crosslinked Ultem 1000 were stable in a wider range of solvents than those on the PAN support. OSN testing in a model system with isomeric alkane solutes verified that manipulated changes to the molecular architecture of the polymer backbone resulted in a higher separation factor between straight and branched alkane isomers.