A general thermodynamic treatment of electrical condensers is presented which avoids the use of thermodynamically ill defined electrostatic quantities and concepts, and which does not neglect surface effects at the condenser plates. The following conclusions emerge. (1) For multicomponent bulk dielectrics, equations similar in form to those obtained previously by Koenig and Frank are derived describing field fractionation etc. (2) The derivations make clear the thermodynamic status of commonly employed electrostatic quantities, including the capacity. (3) Surface effects at the condenser plates are an important aspect of the properties of condensers. (4) By use of an intrinsically reasonable non-thermodynamic assumption an expression in terms of well defined physical quantities is derived for the effect of dielectric composition on the inner potential difference between a metal and a multicomponent dielectric in contact with the metal. This leads to an expression for the absolute inner potential difference between two coexistent dielectrics in contact in the absence of applied fields. (5) The Bridgman assumption concerning the Volta effect is shown to be equivalent to the assumption that the interfacial tension between a conductor and a dielectric is independent of applied field strength at constant temperature and chemical potentials of the dielectric components. (6) Formal results describing the coupling between electrostatic and other forces at microscopic separations are obtained.