The adsorption of isomer-free sodium n-decylbenzenesulfonate-d(4) onto a commercial gamma-alumina sample was investigated in an aqueous solution. The prototype anionic surfactant and positively charged mineral oxide gave a characteristic sinusoidal adsorption isotherm with four discrete regions in the log equilibrium concentration-log adsorption amount plots. Deuterium NMR was applied to probe the molecular motion of the deuterated surfactant adsorbed on the surface. The results were not consistent with either the reverse orientation model or the bilayer model. An intermediate model between the two models was thus proposed for describing a partial occupation of the outer layer of the surfactant aggregates formed in a low concentration range.