AbstractDiffusion coefficients of binary solutions can be measured accurately by observation at long times of isothermal diffusion in a vertical cell closed at the ends (restricted diffusion). The present analysis, which accounts for the effects of solvent flux and variable solution properties, demonstrates that the experiment yields a well‐defined, differential diffusion coefficient, even in concentrated solutions.Observation is effected by Rayleigh interference optics, and results for aqueous potassium chloride solutions confirm the accuracy of the method.