We investigated the adsorption of hydronium ions on Ag(1 1 1) in conditions that simulate the structure of the double layer using the ab initio quantum mechanical Moller-Plesset second-order method. The most representative points of the potential energy surface for bare hydronium on Ag(1 1 1) were first investigated. Then, the ion was hydrated with 1, 2, and 3 water molecules, and the structures of the hydronium-water complexes were studied on Ag(1 1 1) under different externally applied homogeneous electric fields. Bare hydronium adsorbs via the hydrogen atoms with C-3v or C-s symmetry. For these coordinations, the potential energy surface has a small corrugation: the binding energy on the hcp hollow site (-56 kcal/mol) is only 2 kcal/mol more stable than on the ontop site. On the other hand, adsorption via the oxygen atom is destabilized due to the Pauli repulsion with the metal. The equilibrium geometry of the trihydrated complex (H9O4+) has the water molecules located between the hydronium ion and the surface, indicating that hydronium does not specifically adsorb. The surface reaction leading to H9O4+ from adsorbed water and hydronium is very exothermic (-32 kcal/mol) mainly due to the formation of hydrogen bonds. The electric field has a smaller influence on the adsorption of the hydrated ion than on the bare ion due to the screening of the water molecules. The different contributions to the binding energy in the presence of electric fields were considered. The polarization contribution is more important for H9O4+ than for H3O+ and leads to a stabilization of the trihydrated complex at small positive electric fields.