Concentration-dependent binary Flory-Huggins interaction Parameters belonging to the quaternary system poly(ether sulfone) (PES)-poly(vinylpyrrolidone) (PVP)-N-methylpyrrolidone (NMP)-water are determined by means of high-pressure osmometry. The interaction parameter between PVP and water was in accordance with literature values. The interaction parameters between PES and NMP and between PVP and NMP were found to be slightly concentration dependent; the values indicate near-THETA conditions for both polymers in the solvent. The binary interaction parameter between the polymers PES and PVP is estimated from ternary osmotic measurements and found to be quite low : between -0.5 and -1.0. This is in accordance with earlier findings. Cloud point curves were evaluated experimentally and theoretically.1 The agreement between theory and experiment was qualitative.