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. We now need to measure the interaction parameters with measurements at the air/liquid interface. Time was previously allocated for this but was lost due to beam failure.