The technique of specular neutron reflection has been used to study the structure and composition of single and mixed surfactant layers adsorbed from aqueous solution onto self-assembled monolayers of octadecyl trichlorosilane on silicon oxide, The surfactants used were the anionic sodium n-dodecyl sulfate and the zwitterionic n-dodecyl-N,N-dimethylamino acetate and all measurements were made at the critical micelle concentration of the system, Molecular areas for the single surfactants were found to be similar to those measured at the air/solution interface and to those measured at the glycerol trioleate/solution interface, The structure and composition of the mixed surfactant layers are also similar to those obtained at the air/solution interface, The two surfactants mix at the solid/liquid interface in a highly nonideal and specific manner.