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From the scattered light intensities of polydisperse polymer solutions, the thermodynamic parameters of the system, i.e. the chemical potentials of the solvent and the polymer and the interaction parameter in the Flory and Huggins equation, can be determined. The maxima of the scattered light intensity (as a function of concentration) and of the quotient of scattered light intensity and concentration (as a function of concentration) give information on these thermodynamic parameters, and localize the maximum of the spinodal. The spinodal itself can be determined from data in the stable region by means of an extrapolation procedure. These thermodynamic parameters can be determined also from sedimentation-diffusion equilibrium data, even over a considerable range of concentrations.