There is a general cor relation between packing density and molecular diameter for various solvents. Molecular diameters are estimated by three methods, in which two diameters are on the basis of the scaled particle theory (SPT) and the third one is obtained from actual molecular volumes assuming a spherical molecule shape. Hydrophobicity or solvophobicity can be reasonably explained by the large deviation of packing density from a hypothetical solvent with the same size and less cohesive energy density (ced).