Generalized transport coefficients are indispensable for the study of interactions between species of the same kind in binary solutions. Distinct diffusion coefficients are best suited for this purpose because of their finite limiting values and inherent symmetry. Equations are derived for the calculation of these coefficients from experimental data for the mass-, volume-, number-, and solvent-fixed frames of reference. Distinct diffusion coefficients in the 4 reference frames computed for 14 binary nonelectrolyte solutions show that the number-fixed frame is probably best suited for the comparison of the macroscopic properties of these solutions. A point of contention has been the frame of reference of the Friedman-Milis equation;(1) surprisingly, the mass-fixed frame describes its D-ij(d) values closely.