A model for the metal/liquid electrolyte interphase is presented, in which metal is modelled as jellium, the electrolyte as an ensemble of hard spheres. An expression is derived for the interfacial capacity at the potential of zero charge. Numerical model calculations are performed for various metal/solvent systems. The model gives good results for the capacity of second and third row sp metals, and for the temperature dependence of the Hg/water interface. © 1984 American Institute of Physics.