A theoretical discussion of Donnan equilibria, based upon the exact thermodynamic treatment of this subject by Donnan and Guggenheim, is presented. The more familiar terminology of Lewis and Randall is substituted for that of Donnan and Guggenheim. The expression, Donnan equilibria, is applied only to equilibriums between phases upon which measurements can be made and not to equilibriums between microregions in a suspension such as the "micellar" and "intermicellar" regions. For real systems, such approximate generalizations as the ion conc. product principle are not adequate. On the other hand, nothing is gained by substituting the term "activity" for "concentration" in a discussion, unless the quantities which have been measured are actually activities rather than concs. Emphasis is placed upon the conclusions of Guggenheim that, whereas activities of electrolytes are thermodynamically definable, the individual activities of ions are not. Use of the concept of ionic activities should be avoided. The valid thermodynamic relations have been infrequently studied experimentally and, although they are important as criterions for the existence of equilibriums, their consideration does not yield much interesting information. The use of the Donnan principle by pedologists has been essentially qualitative. In absence of quantitative formulations it is doubted that soil properties can be adequately explained in terms of Donnan equilibria in preference to other hypotheses. The importance of the Donnan equilibrium as a regulative principle in soil chemistry has probably been greatly overemphasized. The Donnan equilibrium can be used as an exptl. device in the study of certain soil colloidal properties.