To investigate nanofiltration (NF) separation for recycling polyethylenglycol (PEG) from an ion partition process using an aqueous two-phase system, fractionation performance of five different NF membranes (NF270, SR3, SR100, SR2, and BW30) with solutions of NaNO3, KClO4, and PEG 4000 in water comprising various mixtures were studied. PEG rejections and salt passage were analyzed and explained based on size exclusion as well as electrostatic interactions. The highest permeate flux at high rejection of PEG as well as the lowest salt rejections were obtained with SR2 and NF270 membranes. Similar salt rejections were observed for mixed solute solutions and complex mixtures, all following this trend: SR3>NF270>SR2. The PEG rejections were well above 95%. This study also revealed that high salt passage of above 90% could be achieved with the same NF membrane only by unstirred conditions through concentration polarization mechanism; however, at the expense of low flux, especially with high PEG concentrations.