Multicomponent diffusion coefficients at three compositions of the ternary system polystyrene-toluene-cyclohexane were measured at 28.00° with a Gouy interferometer. The strong concentration dependence of the diffusion coefficients was removed by repeating all experiments at the same average concentration but different concentration differences and extrapolating the results to zero concentration difference. That the polymer behaves as a single species is shown by binary data. The multicomponent diffusion coefficients measured show the largest deviations from Fick's law yet observed. Because of these large deviations, the data offer the opportunity of evaluating multicomponent flux equations. The two most promising forms are briefly discussed, stressing the restraints on the various coefficients. Experimentally, they are compared as to accuracy of measurement, variation with composition, and molecular significance.