A model for the adsorption of ionic surfactants on an oppositely charged solid surface of uniform charge density is developed. The model is based on the consideration that, on the solid surface, adsorbed surfactant monomers, monolayered and bilayered surfactant aggregates of various sizes, and "empty sites" constitute a nonideal two-dimensional mixture. The competition between the entropic and enthalpic contributions to the free energy is responsible for the formation of the monolayered or bilayered surfactant aggregates. The nonideality is treated via the Flory-Huggins equation extended to a two-dimensional solution. The standard free energy change is calculated by considering five contributions : the hydrophobic effect and the conformational change of the surfactant tails, the electrostatic and steric interactions due to the surfactant headgroups, and the surfactant tails-water interfacial free energy. The electrostatic effects are treated in the framework of the Poisson-Boltzmann equation. The model, which does not include empirical parameters, is compared with the experimental adsorption isotherms for three sodium alkylbenzenesulfonates on alumina and kaolinite and for sodium dodecyl sulfate on alumina.