A general statistical mechnical model of the adsorption of ions at the electrode interface is presented. The three-dimensional adsorption model is equivalent to a two-dimensional lattice gas. The adsorption isotherm for a single ion is given in terms of Fade approximants for the high and low fugacity limits. This formalism is applied to the case of the underpotential deposition of copper onto gold(lll). The spikes in the voltammogram of this deposition are viewed as resulting from a first order phase transition on a honeycomb lattice followed by a first order phase transition on a triangular lattice. This model is extended by incorporating the coadsorption of(bi)sulfate with copper and by considering longer range interactions between adatoms using the mean neighbor approximation. The model voltammograms agree quite well with the experimental ones.