Adsorption characteristics of organic compounds on an octadecylsilyl (ODS)-silica gel for two mobile phases, i.e., methanol/water (70/30, v/v) and acetonitrile/water (70/30, v/v) are compared. It is confirmed that the values of adsorption equilibrium constant, isosteric heat of adsorption, and activation energy of surface diffusion are smaller in 70 vol% acetonitrile than in 70 vol% methanol. On the contrary, the values of surface diffusion coefficient, D-s, are larger in 70 vol% acetonitrile than in 70 vol% methanol. These results indicated that the interaction between ODS ligands and solute molecules was weaker in 70 vol% acetonitrile than in 70 vol% methanol. The ratio of D-s to molecular diffusivity, D-m, is about 0.3 irrespective of chromatographic conditions, i.e., the type of organic modifiers and sample compounds, and temperature. It is possible to calculate D-s from D-m, which can be estimated by various correlations previously proposed in the literature. The almost constant ratio of D-s to D-m suggests the presence of a kind of correlation between surface and molecular diffusions.