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Specular neutron reflection has been used to determine the structure and composition of the mixed ionic-non-ionic surfactants adsorbed at the hydrophilic silica solid/water interface. Measurements on two different mixed surfactant systems are reported: the cationic/nonionic mixture of hexadecyltrimethyl ammonium bromide, C16TAB, and hexaethylene glycol monododecyl ether, C12E6, and the anionic/nonionic mixture of sodium dodecyl sulfate, and C12E6. For the C16TAB-C12E6 mixture, pH is shown to have a dramatic effect on the relative affinity for adsorption of the two surfactants at the interface.