The diffusion coefficients of the ternary system pentaethylene glycol-triethylene glycol-water have been measured at five compositions at constant total solute concentration. A theory that is an extension of the current hydrodynamic equations for calculation of D-ij has been tested on this ternary system. These equations use values of what we will refer as the "effective" volumes of the solutes. These volumes were obtained by the analysis of the diffusion coefficients of the two corresponding binary systems. The agreement between theory and the experimental values for the main-term diffusion coefficients D-ii is quite good and the agreement between the predicted and measured cross-term diffusion coefficients D-ij (i not equal j) is very encouraging. The solute-solute nonbonding interactions seem not to have a significant effect on the diffusion coefficients in our experimental conditions.