Organic solvent nanofiltration is an emerging technology made possible by the recent development of solvent resistant nanofiltration (SRNF) membranes. These membranes have many potential applications from continuous operation over many months in refinery systems [1,2] to short term operation for a few hours in batch chemical processes [3]. In this paper, solvent flux decline and membrane separation properties are investigated (including their dependence on pressure), using methanol with quaternary alkyl ammonium bromide salts with molecular weights (MW) in the range 322 to 547 Daltons as solutes. The membranes are characterised in terms of an equivalent uniform pore size using three simple pore flow models: Ferry model, Steric Hindrance Pore (SHP) model and Verniory model.