The interparticle surface separation in mono- and polydisperse concentrated suspensions of spherical particles is discussed. For monosized particles, it is shown that the "expansion" argument of Frankel and Acrivos (1967) can be extended from cubic to arbitrary packing geometries. This argument is then further extended to polydisperse suspensions, resulting in an equation of the same form, but incorporating the surface mean particle diameter for the interparticle surface separation in terms of the packing fraction of the equivalent packed bed (a packed bed with the same packing geometry as the suspension) and that of the suspension. Another approach to interparticle surface separation in suspensions (the "volume of free liquid" argument) confirms the usefulness of the surface mean particle diameter for polydisperse suspensions. A relation potentially useful for suspensions of nonspherical particles is also given.