Small angle neutron scattering and neutron reflectivity have been used to study the adsorption and micellization of the mixed anionic-nonionic surfactant system, sodium dodecyl sulfate (SDS)/n-dodecyl hexaethylene glycol (C(12)E(6)) in 0.1 M NaCl. The composition of the surfactant layer adsorbed at the air-solution interface is compared with the micellar composition over a range of concentrations, extending from 10 to 100 times the critical micellar concentration(cmc). It is found that the results are in close agreement with the predictions of regular solution theory (RST).