The concept of ion-free layer thickness is examined on the basis of contemporary theories of the electrical double layer. A comparison between surface excesses derived from Modified Poisson-Boltzmann (MPB and URMPB) and mean field (GC and UDCA) theories is performed. The influence of unequal distances of closest approach for cations and anions in the analysis of experimental surface excesses is cancelled to a significant extent by inclusion of self-atmosphere and exclusion volume effects. An application is made to the mercury/aqueous LiCl, NaCl and KCl interfaces.