Ternary aqueous solutions of two water soluble polymers are separated into two coexisting phases above a given concentration: one of the aqueous phases will be enriched in one of the polymers while in the other phase there is prevalence for the second polymer. The biaqueous nature of the two-phase system as well as difference in their properties makes it possible to use them for the partitioning and separation of biological materials such as cells, organelles, enzymes, proteins, etc [ 1-3]. Although both polymers are water-soluble, they separate into two coexisting phases and both equilibrium phases have same components with different concentrations. Therefore aqueous biphasic systems (ABS) fo rmation is an unusual phenomenon and although this phenomenon is well documented and has been extensively investigated in the literature, its mechanism at the molecular level is still unclear. In this work, in an attempt to gathering further information to evaluate the recently proposed [ 4] mechanism of ABS formati on, isopiestic equilibrium molalities of several ternary aqueous solutions of two water soluble polymers capable or not of inducing phase separation were performed at 298.15 K in both one-phase and two-phase areas.
