The binding of nonionic and anionic surfactants from bulk solutions onto preadsorbed layers of bovine serum albumin (BSA) at the hydrophilic silicon oxide/water interface has been studied using specular neutron reflection. The distributions of surfactant, protein, and water at the solid/aqueous interface were separately determined by using deuterium labeling of the surfactant and water. The preadsorbed BSA layers were formed by equilibrating the hydrophilic oxide surface with a 0.15 g dm(-3) BSA solution, which produced a densely packed uniform BSA layer of thickness 35 +/- 3 Angstrom. The possible association of nonionic surfactant pentaethylene glycol monododecyl ether (C-12 E-5) with BSA at the interface was examined by following the change in neutron reflectivity before and after surfactant addition. That no measurable difference between these reflectivity profiles was observed suggests that the nonionic surfactant did not bind to the adsorbed BSA. The binding of sodium dodecyl sulfate (SDS) onto the BSA layer was studied over a wide SDS concentration range. Noticeable binding was detected at an SDS concentration of 1 x 10(-4) M, and as the SDS concentration increases the thickness of the mixed layer also increases. At [SDS] 1 x 10(-3) M the reflectivity profile from the mixed layer became identical to that between the bare oxide/D2O interface, suggesting that at this SDS concentration the mixture is completely removed from the interface. The surface excesses of BSA and SDS, and the structural distributions of BSA, SDS and water at each SDS concentration were obtained by simultaneously fitting a single structural model to the set of measured neutron reflectivity profiles. The results show that at low SDS concentrations the structural profiles of the interfacial components are well approximated as uniform layer distributions and as SDS concentration is increased the distributions become unsymmetrical. The binding of SDS results in an expansion of the preadsorbed BSA layer from 35 +/- 3 Angstrom in the absence of SDS to some 80 Angstrom at 3 x 10(-4) M, suggesting considerable structural deformation of the protein. The volume ratio of SDS to BSA in the mixed layer was found to be 0.45, in close agreement with the literature value for the binding of SDS onto denatured protein in the bulk, suggesting that the protein in the adsorbed complex is also denatured.