Relationships between the activation energies (QA and QB) of self-diffusion of each component and the concentration CB of a completely disordered binary solid solution are derived on the basis of a quasi-chemical model. These theoretical results are compared with experimental data.When the atomic size of the two components is approximately the same, a linear relationship is obtained between(QB - QA) and CB for both f.c.c. and b.c.c. structure. This is in good agreement with the experimental results of Ag-Au system.In the low concentration range, the relationship between the activation energy QA of self-diffusion of the solvent and the concentration CB is also a straight line. The sign of the slope can be predicted from the relative magnitude of activation energy of self-diffusion of the solvent and that of impurity diffusion in the same solvent. This prediction is in good agreement with the known experimental data of self-diffusion of silver in nine different silver alloys of low alloy concentration.