Ionic charge and size of the species present govern the selectivity of surface adsorption of counterions in foam fractionation processes. A theory based on the Gouy-Chapman model of the diffuse double layer, with the restriction that the closest approach to the surface is determined by the finite size of the hydrated ions, enables one to predict the distribution factor of each species between a solution of mixed electrolytes and a surface layer, and to calculate the selective adsorption coefficient between two ions. Good agreement was found between the theoretical prediction and experimental data.