Characteristics of surface diffusion phenomena were studied under low-loading or analytical conditions of reversed-phase Liquid chromatography. As mobile phases, methanol/water and acetonitrile/water mixtures were used. In both systems, several similar relationships were confirmed for the enthalpy-entropy compensation effect, the linear free-energy relation, and some correlations between the surface diffusion coefficient, D-s, and mobile-phase composition, phi, and between the ratio of D-s to molecular diffusivity and phi. Similarity of the correlations indicated that the surface diffusion mechanism was essentially identical irrespective of the type of the organic modifiers. A strong influence of phi on D-s was confirmed. A nomograph concerning corresponding mobile-phase compositions for D-s as well as for adsorption equilibrium constant was proposed for the both mobile-phase systems. On the basis of the several correlations, D-s under various conditions in low-concentration systems can be estimated.