Asymmetric membranes for organic solvent nanofiltration (OSN) were fabricated from polyimides (PI) synthesised by polycondensation reactions. The effect of molecular weight and chemical structure of the synthesised PIs on the PI OSN membrane formation and performance was investigated. This work is the final part of a three parts study seeking a better understanding of how membrane formation parameters affect membrane performance. During a co-polymerisation reaction 3,3',4,4'-benzophenone tetracarboxylic dianhydride was reacted with various diisocyanates i.e. toluene-2,4-diisocyanate, 4,4'-methylenebis(phenyl isocyanate), 3,3'-dimethoxy-4,4'-biphenyl diisocyanate, and isophorone diisocyanate. It has been shown that the rejection of PI OSN membranes is not influenced by the molecular weight of the PI which varied between 30,000 and 70,000 Da. However, there is a minimum molecular weight (35,000 Da) required to cast defect free membranes with good mechanical strength for OSN applications. Likewise, too high PI molecular weight (80,000 Da) results in too viscous dope solution which hinders membrane casting. Structural changes in the PI and the method of polymerisation (block vs random) were found to affect both flux and rejection of the membranes. The use of the random PI resulted in the formation of more open membranes having lower rejection and higher flux. (C) 2011 Elsevier B.V. All rights reserved.