The phase diagrams and equilibrium concentrations of polyethylene glycol (PEG)/Dextran/Water and Polyethylene glycol/Ammonium sulfate/Water systems with different molecular weights of PEG were experimentally determined by measuring the concentration of PEG, dextran and/or ammonium sulfate. In phase diagrams of PEG-water with ammonium sulfate or dextran the addition of salt or dextran induced the phase separation with different partitioning of PEG, but salt and dextran were accumulated in aqueous media in any cases. The increase in molecular weight of PEG broadened the phase separation region in both cases. In PEG/Water/Dextran or Ammonium Sulfate systems, the partition coefficients of egg albumin determined in a column of sephadex G-75 gel increase with concentrations of dextran, but those in ammonium sulfate showed apparently maximum behavior, suggesting there exist some limiting conditions in protein separation. In calculating the phase diagram, on the other hand, the partitioning of dextran as a model protein, the blob model developed by Ban and Kim, consisting of pseudo-components of blobs for polymer segments and local composition model showed a good agreement with experiments.