Ternary diffusion coefficients at 25° for one composition of the system water-sucrose-potassium chloride have been measured both with the Gouy diffusiometer and with the diaphragm cell. The same solutions were used for both types of experiments to ensure measurement of coefficients averaged over the same range of concentrations; at the completion of each diaphragm cell experiment the differences in both the refractive index and the density of the solutions in each cell were measured. By combining these data with the initial values of the refractive index and density differences, the four ternary diffusion coefficients were obtained by a new analytical technique which is free from the experimentally inconvenient restrictions of earlier theories. The diffusion coefficients measured by each method agree within the estimated error although the estimated error for the diaphragm cell experiments is roughly four times the error for the Gouy experiments. The coefficients based on measurements of both refractive index and density differences showed significantly smaller errors than those based on either refractive index or density measurements.