The retention properties of an ultrafiltration (UF) membrane with 1000 Da molecule weight cut-off (MWCO) were investigated using bisphenol A (BPA) as a model hydrophobic solute under the influence of different electrolyte ions and varying pH. The Stokes radius of the hydration molecule of the BPA was calculated with the hydrodynamic model by simulation of BPA transportation in the membrane. The rejection efficiencies of BPAs were found to visibly decrease in the presence of various ions (Na+, Li+, Mg2+, H+ and OH-), while simulation results showed that the effective radius of the hydration molecules reduced accordingly. The lowering of the BPA rejection efficiency induced by ions followed the laws of Hofmeister series, i.e., Mg2+ > Li+ > Na+ and H+ > OH-. It is proposed that the added ions may compete water with BPA hydration molecules and cause their partial dehydration, which causes a decrease in the effective radius of the BPA molecules. This research bolsters our understanding on the effects of hydration and dehydration on the removal of hydrophobic emerging contaminants in UF membranes with low MWCO. (C) 2015 Elsevier B.V. All rights reserved.