One of the main difficulties in studying the interaction of protein and a surfactant at an interface is to establish the structural distributions of each component in the mixed layer. We demonstrate in this work that the distributions of the surfactant, protein, and water adsorbed at the hydrophilic solid/aqueous solution interface can be separately and unambiguously determined by using specular neutron reflection combined with deuterium labeling of the surfactant and water. Adsorption of bovine serum albumin (BSA) from a 0.15 g dm(-3) solution onto a hydrophilic silicon oxide surface produced a densely packed uniform layer with a thickness of 35 +/- 3 Angstrom. The binding of sodium dodecyl sulfate (SDS) onto this preadsorbed BSA layer was studied at a constant SDS concentration of 1 x 10(-4) M with different isotopic compositions of SDS and water. The surface excesses of BSA, SDS, and the structural distributions of BSA, SDS, and water were obtained by simutaneously fitting a single structural model to the set of measured neutron reflectivity profiles. The results show that at this SDS concentration the structural profiles of the interfacial components are approximated well as uniform layer distributions. The binding of SDS results in an expansion of the preadsorbed BSA layer from 35 +/- 3 Angstrom in the absence of SDS to 50 +/- 5 Angstrom, suggesting considerable structural deformation of the protein. The weight ratio of SDS to BSA in the mixed layer was found to be 0.43, 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.