The liquid-liquid equilibria (LLE) for the dextran-poly(ethylene glycol)-water aqueous two-phase systems with various polymer molecular weights were measured at 20-degrees-C. The molecular weight distributions of dextran (DEX) and poly(ethylene glycol) (PEG) in the PEG-rich phase (top phase) and DEX-rich phase (bottom phase) were also measured. To take into account deviations from random mixing, the interaction term of the conventional Flory-Huggins equation was empirically modified. The molecular weight distribution of DEX was expressed by a GAMMA distribution function because the DEX sample used showed a wide molecular weight distribution though the PEG samples were almost monodispersed. LLE and molecular weight distributions were correlated successfully using a continuous thermodynamic approach.