To facilitate model studies of the double layer at charged surfaces, an analysis is made of the thermodynamics of the dissociation phenomena which take place at mineral oxide surfaces. Two alternative equations may be used to describe the dependence of the surface potential and charge upon the concentration of potential determining ions in the solution. The two different approaches are similar to those given previously1,2 but analysis is made of the specific adsorption potential which is expressed in a form which permits it to be theoretically calculated. While specifically developed for insoluble oxides, the theory is equally applicable to other systems.