The water/polymer interaction parameter plays an important role in the thermodynamic description of membrane-formin g water/solvent/polymer solutions with the Flory-Huggins (FH) model. In order to discuss reasons for the wide variety of values for this parameter reported in the literature, interaction parameter was determined utilizing two different methods for polysulfone, poly(ether sulfone), cellulose acetate and poly(ether imide) (PSU, PES, CA, and PEI). In a "direct" determination, water sorption isotherms have been measured and analyzed with the FH model. In a second, "indirect", method an optimal parameter value was found by fitting experimental cloud point data of ternary polymer systems with the FH model. Liquid-liquid equilibrium (LLE) calculations have been carried out under the assumption that the water/solvent and solvent/polymer interactions are correctly described by known composition dependent interaction parameters. For a given polymer, the water/polymer interaction parameters resulting from fits of ternary systems with different solvents agree well. But the obtained values are smaller than the ones resulting from the sorption experiment. The difference is smaller for a more hydrophilic polymer with a higher water uptake than for hydrophobic polymers with a lower uptake. Reasons for this difference will be discussed. (c) 2005 Elsevier B.V. All rights reserved.