A model for the adsorption of ionic amphiphiles in the form of a bilayer at an oppositely charged surface in presented. The treatment is based on an earlier model developed by Jonsson and Wennerstrom for the association of ionic amphiphiles in aqueous solution. The electrostatic effects are treated by using the Poisson-Boltzmann equation. In addition to the electrostatic interaction, the influence of dispersion forces, solvation, and steric interaction on the interaction between the bilayer and the surface are also considered. From the model expression for the free energy of the system it is possible to calculate the critical amphiphile concentration for formation of a bilayer at the surface and the concentration dependence of the thickness of the bilayer. It is found that the critical concentration strongly depends on the charge density of the surface; at low charge densities the electrostatic interaction between the surface and the bilayer is too small to stabilize the bilayer at amphiphile concentrations below the cmc. The outcome of the model is compared with experimental results obtained by in situ ellipsometry for the alkyltrimethylammonium bromide/SiO2 system. It is concluded that at low to neutral solution pH it is questionable if a bilayer is formed at the surface. At high solution pH, implying high charge densities of the surface, the bilayer adsorption model is able to describe the effect of changes of the electrolyte concentration, co-ion valence, and alkyl chain length on the adsorption of alkyltrimethylammonium bromides on SiO2.