The effect of the molecular volume on the diffusion coefficients has been investigated for ternary systems. Mutual diffusion coefficients, D-ij, for the ternary system tetra(ethylene glycol)-di(ethylene glycol)-water at eight mean compositions were measured by Gouy interferometry at 25 degreesC. The data show the effect of the total concentration on the main- and cross-term diffusion coefficient values. We used these data to check, at high concentration, the predictive values of our recent semiempirical equations for nonelectrolyte solutions. The differences between the experimental and predicted diffusion coefficients found at the higher concentrations are discussed in terms of solute-solute nonbonding interactions.