A model Hamiltonian for electron transfer from a metal electrode to a solvated reactant via a metallic adsorbate is proposed. The adsorbates are distributed randomly over the electrode surface, and a coherent-potential approximation has been employed to treat this randomness. Both the adsorbates and the reactant are assumed to interact with the solvent, which is modeled as a bath of phonons with frequencies in the classical regime. Both the adiabatic and the nonadiabatic potential energy surfaces are calculated, and their dependence on the adsorbate coverage is highlighted. In the low coverage regime the potential-energy surfaces exhibit features similar to a bridge-assisted electron transfer reaction, whereas for higher coverages the surfaces resemble those for direct heterogeneous transfer. This change of shape is caused by the metallization of the adsorbate layer at higher coverages. (C) 2004 American Institute of Physics.