Spun-cast layers of polystyrene have been transferred to large silicon crystals as substrates for neutron reflectivity studies and to prisms for infrared-attenuated total reflection measurements of surfactant adsorption. This paper describes an investigation of the adsorption from aqueous solutions of sodium dodecyl sulfate (SDS) to this interface of aqueous solution and polystyrene. The structure and amount of adsorbate is described and discussed. The adsorption isotherm shows that both the thickness and volume fraction of the SDS layer increase to plateau values above the critical micelle concentration (cmc) where the area per molecule is approximately 42 Angstrom(2). Above the cmc, there is a decrease in the total amount of adsorbed material unless SDS of very high purity is used. This unusual behavior is explained by the presence of dodecan-1-ol from hydrolysis of SDS which is highly surface active but can be solubilized in micelles of SDS.