Two-color optical measurements of thermodiffusion in ternary mixtures frequently suffer from ill-conditioned contrast factor matrices, whose inversion leads to very large experimental errors. In this contribution weshow how the error amplification can be avoided in situations where a priori knowledge about the directionsof the eigenvectors of the diffusion matrix is available. We present optical beam deflection experiments onsolutions of the polymer polystyrene of Mw = 4880 g/mol in a mixed solvent of toluene and cyclohexane. Inthis system the two diffusion eigenvalues differ by almost one order of magnitude. The large eigenvalue canbe attributed to the interdiffusion of the two solvents and the small one to the polymer diffusion relative tothe mixed solvent. The pre-selection of the eigenvectors renders the method stable against fluctuations ofthe experimental parameters. Both the diffusion and the Soret coefficients attributed to the two modes agreevery well with the respective values of corresponding binary mixtures.
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