Specular neutron reflection has been used to determine the structure and composition of the mixed cationic and nonionic surfactants of hexadecyltrimethylammonium bromide, C(16)TAB, and hexaethylene glycol monododecyl ether, C12E6, adsorbed at the hydrophilic silica solid/water interface. Measurements have been made at two different values of pH (2.4 and 7.0), where the relative affinity for the adsorption of the two surfactants at the interface is dramatically altered. At high pH (pH = 7.0) the composition of the adsorbed bilayer is dominated by C(16)TAB and the structure of the bilayer is nonuniform : that is, the surfactant layer immediately adjacent to the solid silica surface is richer in C(16)TAB. At the lower pH (pH = 2.4), where the affinity for the surface of C12E6 is much greater relative to that of C(16)TAB, the surface composition is closer to the bulk solution composition and the structure of the adsorbed bilayer is essentially uniform.