The overall objective is to model the amount and composition of adsorbed mixtures of three types of zwitterionic surfactant and two charged surfactants at the silica/water interface in terms of the regular solution model, which hitherto has only been appplied to the air/liquid surface. Measurements on dodecylsulphobetaine mixtures with SDS and with C12TAB show features which confirm the suitability of this system for a more complete study. In particular, there is an adsorption maximum in the mixture with SDS and the mixture with C12TAB shows anomalously low adsorption with a minimum. We now need to measure the composition of selected concentrations at the solid/liquid interface.