The liquid-liquid equilibria of the aqueous two-phase systems consisting of polyethylene glycol (M(w) = 7,500) (1)-dipotassium hydrogenphosphate (2)-water(3), polyethylene glycol(l)- ammonium sulfate(2)- water(3) and polyethylene glycol(1)- dextran (M(w) = 515,000, M(n) = 195,300) (2) - water (3) were measured at 25-degrees-C. The partition coefficients of 15 benzene derivatives in these aqueous two-phase systems were also determined at 25-degrees-C. Further, the osmotic virial equation was used to correlate the liquid-liquid equilibria of the aqueous two-phase systems and the partition coefficients of 15 benzene derivatives. An estimation of the partition coefficients was attempted by using molecular connectivity indices.