The stability of poly(vinyl chloride)-based polymer inclusion membranes (PIMs) containing 40%(w/w) Aliquat 336 (R3MeN+Cl-) and 60%(w/w) PVC is studied in detail when these membranes are immersed in deionized water and in aqueous solutions containing various salts or hydrochloric acid. It is shown that the mass loss is suppressed in the salt solutions and that the effectiveness of the suppression follows the order: NaClO4 > NaNO3 > NaSCN > NaCl approximate to LiCl approximate to HCl > Na2SO4 > CH3COONa. It is also demonstrated that the distribution ratio of R3MeN+X- (where X- is Cl- or NO3-) between the PIM and the aqueous solution depends on the concentration of the "common anion" X- in the solution. However, in every case, the PIM eventually reaches an equilibrium with the aqueous solution, and after that the PIM composition remains stable. The ion-exchange equilibrium constants between Cl- on one hand and NO3- or SO42- on the other are reported to be Kie-NO3 = 1.8 x 10(1) and Kie-SO4 = 1.5 x 10(-2), respectively. The results reported in this study suggest that although PlMs are capable of losing some membrane liquid phase when exposed to aqueous solutions, this loss can be minimized or even eliminated by increasing the solution concentration of the common anion. (c) 2012 Elsevier B.V. All rights reserved.