Ternary diffusion coefficients at 25°C. of the system 0.5 g./cc. d‐tartaric acid—0.5 g./cc. l‐tartaric acid‐water are reported. The results allow experimental verification of the theory for the diffusion of two indistinguishable solutes and clearly illustrate the difference between mutual and tracer diffusion. Moreover, the results allow calculation of ternary mass transfer coefficients and ternary tray efficiencies for the case of two similar species dissolved in a third.